acinclude: better fix for pkg-config 0.28 behaviour

Old pkg-config always retured a non-empty string for --cflags.
It always contained at least one bogus space. New pkg-config
does not do that. Do not rely on --cflags being non-empty.

acinclude.m4

@@ -317,6 +317,7 @@ if $PKG_CONFIG --exists "$2" ; then
 	AC_MSG_RESULT(yes)
 	$1_CFLAGS=$($PKG_CONFIG --cflags "$2")
 	$1_LIBS=$($PKG_CONFIG --libs "$2")
+	$1_EXISTS=yes
 else
 	AC_MSG_RESULT(no)
 fi
@@ -327,7 +328,7 @@ AC_SUBST($1_LIBS)
 
 AC_DEFUN([TUXBOX_APPS_LIB_PKGCONFIG],[
 _TUXBOX_APPS_LIB_PKGCONFIG($1,$2)
-if test -z "$$1_CFLAGS" ; then
+if test x"$$1_EXISTS" != xyes; then
 	AC_MSG_ERROR([could not find package $2]);
 fi
 ])