Allow TQt3 includes to exist somewhere other than /usr/include/tqt

The test for TQt3 fails when it's not installed in /usr due to CXXFLAGS being hardcoded to /usr/include/tqt. Amended to append to CXXFLAGS.

Signed-off-by: aneejit1 <aneejit1@gmail.com>
(cherry picked from commit ae96f3ea48)

acinclude.m4.in

@@ -1233,7 +1233,7 @@ AC_DEFUN([KDE_USE_QT],
   AC_LANG_SAVE
   AC_LANG_CPLUSPLUS
   ac_cxxflags_safe="$CXXFLAGS"
-  CXXFLAGS="-I/usr/include/tqt"
+  CXXFLAGS="$CXXFLAGS -I/usr/include/tqt"
   AC_TRY_COMPILE([
   #define TQT_VERSION_ONLY
   #include <tqt.h>