configure: rework acinclude.m4

Added more documented output for users and removed some unused
lines (such as assignments after AC_MSG_ERROR, which do not make sense).
Additionally, more checks have been added with result output.

DEBUG_CFLAGS can now be used as an environment variable.
This change was made because for debugging purposes, users should be
able to adjust the options, rather than relying solely on static options
such as -g3 -ggdb. This is especially important since -O2 may often
not be sufficient.

Do we really need the CDK mode? Would it be a better idea to
handle this via the usual configuration or build system, using
environment variable assignments, or catching specific cases
in a makefile? Additionally, I have the impression that hardly
anyone is using the CDK mode.

acinclude.m4

@@ -4,81 +4,127 @@ AM_MAINTAINER_MODE
 
 AC_GNU_SOURCE
 
+AC_MSG_CHECKING(target)
 AC_ARG_WITH(target,
-	AS_HELP_STRING([--with-target=TARGET], [target for compilation [[native,cdk]]]),
+	AS_HELP_STRING([--with-target=TARGET], [Target mode for compilation, either "native" or "cdk", keep empty for native]),
 	[TARGET="$withval"],
 	[TARGET="native"])
 
+AC_MSG_CHECKING([targetroot (target system root)])
 AC_ARG_WITH(targetroot,
-	AS_HELP_STRING([--with-targetroot=PATH], [the target root (only applicable in native mode)]),
+	AS_HELP_STRING([--with-targetroot=PATH], [The target root (only applicable in native mode.]),
 	[TARGET_ROOT="$withval"],
 	[TARGET_ROOT="NONE"])
 
 AC_ARG_WITH(targetprefix,
-	AS_HELP_STRING([--with-targetprefix=PATH], [prefix relative to target root, e.g. /usr (only applicable in cdk mode)]),
+	AS_HELP_STRING([--with-targetprefix=PATH], [Prefix relative to target root, e.g. /usr (only applicable in cdk mode)]),
 	[TARGET_PREFIX="$withval"],
 	[TARGET_PREFIX="NONE"])
 
 AC_ARG_WITH(debug,
-	AS_HELP_STRING([--without-debug], [disable debugging code @<:@default=no@:>@]),
+	AS_HELP_STRING([--without-debug], [Disable debugging code @<:@default=no@:>@]),
 	[DEBUG="$withval"],
 	[DEBUG="yes"])
 
+AC_MSG_CHECKING(debug enabled)
 if test "$DEBUG" = "yes"; then
-	DEBUG_CFLAGS="-g3 -ggdb"
+	AC_MSG_RESULT([$DEBUG])
 	AC_DEFINE(DEBUG, 1, [enable debugging code])
+
+	AC_MSG_CHECKING(debug flags)
+	if test "$DEBUG_CFLAGS" = ""; then
+		DEBUG_CFLAGS="-g3 -ggdb"
+		AC_MSG_RESULT([Environment variable DEBUG_CFLAGS is not set, default debug flags will be used: $DEBUG_CFLAGS])
+	else
+		AC_MSG_RESULT([Defined by environment variable DEBUG_CFLAGS: $DEBUG_CFLAGS])
+	fi
+else
+	AC_MSG_RESULT([$DEBUG])
 fi
 
 AC_MSG_CHECKING(target)
+if test "$TARGET" = "native" || test "$TARGET" = "cdk"; then
+	AC_MSG_RESULT($TARGET)
+else
+	AC_MSG_ERROR([Invalid target '$TARGET'. Use either 'native' or 'cdk' or leave empty for native.])
+fi
 
 if test "$TARGET" = "native"; then
-	AC_MSG_RESULT(native)
+	AC_MSG_CHECKING(prefix)
-
 	if test "$prefix" = "NONE"; then
 		prefix=/usr/local
 	fi
+	AC_MSG_RESULT($prefix)
 
+	AC_MSG_CHECKING([targetroot])
 	if test "$TARGET_ROOT" = "NONE"; then
-		AC_MSG_ERROR([invalid targetroot, you need to specify one in native mode])
 		TARGET_ROOT=""
+		AC_MSG_RESULT([Is not set, use default target system root: empty])
+	else
+		AC_MSG_RESULT([user defined target system root: $TARGET_ROOT])
+	fi
+
+	AC_MSG_CHECKING([targetprefix])
+	if test "$TARGET_PREFIX" = "NONE"; then
+		TARGET_PREFIX=
+		AC_MSG_RESULT([Is not set, use default target prefix relative to target root: empty])
+	else
+		AC_MSG_RESULT([$TARGET_PREFIX])
 	fi
-	TARGET_PREFIX=$prefix
 
 	if test "$CFLAGS" = "" -a "$CXXFLAGS" = ""; then
 		CFLAGS="-Wall -O2 -pipe $DEBUG_CFLAGS"
 		CXXFLAGS="-Wall -O2 -pipe $DEBUG_CFLAGS"
 	fi
 elif test "$TARGET" = "cdk"; then
-	AC_MSG_RESULT(cdk)
+	AC_MSG_CHECKING(prefix in cdk mode)
-
 	if test "$prefix" = "NONE"; then
-		AC_MSG_ERROR([invalid prefix, you need to specify one in cdk mode])
+		AC_MSG_ERROR([$prefix invalid prefix, you need to specify one in cdk mode])
 	fi
+	AC_MSG_RESULT($prefix)
 
+	AC_MSG_CHECKING([targetroot in cdk mode])
+	OLD_TARGET_ROOT=$TARGET_ROOT
 	TARGET_ROOT=""
-	if test "$TARGET_PREFIX" = "NONE"; then
+	if test "$TARGET_ROOT" = "NONE"; then
-		AC_MSG_ERROR([invalid targetprefix, you need to specify one in cdk mode])
+		AC_MSG_NOTICE([Is not set "$OLD_TARGET_ROOT", using fallback to default: empty])
-		TARGET_PREFIX=""
+
+	else
+		AC_MSG_NOTICE([Is set to "$OLD_TARGET_ROOT", but using default: empty])
 	fi
 
+	AC_MSG_CHECKING([targetprefix in cdk mode])
+	if test "$TARGET_PREFIX" = "NONE"; then
+		TARGET_PREFIX=""
+		AC_MSG_RESULT([default target prefix relative to target root: empty])
+	else
+		AC_MSG_RESULT([user defined target prefix relative to target root: $TARGET_PREFIX])
+	fi
+
+	AC_MSG_CHECKING([gcc/g++ in cdk mode])
 	if test "$CC" = "" -a "$CXX" = ""; then
-		AC_MSG_ERROR([you need to specify variables CC or CXX in cdk mode])
+		AC_MSG_ERROR([you need to specify variables CC and CXX in cdk mode in your envirionment])
 	fi
+	AC_MSG_RESULT($CC / $CXX)
+
+	AC_MSG_CHECKING([CFLAGS and CXXFLAGS in cdk mode])
 	if test "$CFLAGS" = "" -o "$CXXFLAGS" = ""; then
-		AC_MSG_ERROR([you need to specify variables CFLAGS and CXXFLAGS in cdk mode])
+		AC_MSG_ERROR([you need to specify variables CFLAGS and CXXFLAGS in cdk mode in your envirionment])
 	fi
-else
+	AC_MSG_RESULT(CFLAGS: $CFLAGS)
-	AC_MSG_RESULT(none)
+	AC_MSG_RESULT(CXXLAGS: $CXXFLAGS)
-	AC_MSG_ERROR([invalid target "$TARGET", choose "native" or "cdk"]);
 fi
 
-AC_DEFINE_UNQUOTED([TARGET], ["$TARGET"], [target for compilation])
+AC_MSG_CHECKING(exec_prefix)
-AC_DEFINE_UNQUOTED([TARGET_ROOT], ["$TARGET_ROOT"], [native: the target root; cdk: empty])
-AC_DEFINE_UNQUOTED([TARGET_PREFIX], ["$TARGET_PREFIX"], [native: auto-assigned path; cdk: path relative to target root])
-
 if test "$exec_prefix" = "NONE"; then
 	exec_prefix=$prefix
 fi
+AC_MSG_RESULT($exec_prefix)
+
+AC_DEFINE_UNQUOTED([TARGET], ["$TARGET"], [target for compilation])
+AC_DEFINE_UNQUOTED([TARGET_ROOT], ["$TARGET_ROOT"], [root path at the target system native: the target root; cdk: empty])
+AC_DEFINE_UNQUOTED([TARGET_PREFIX], ["$TARGET_PREFIX"], [native: auto-assigned path; cdk: path relative to target root])
+
 
 AC_CANONICAL_BUILD
 AC_CANONICAL_HOST