configure.ac: add m4 macros that can check for compiler option support

We now use this to set certain options, if supported by the compilers
(C/C++).  Currently we enable various options, like
-Wno-format-truncation.  We use this to ignore complaints about
possibly truncations due to the use of the snprintf() family of
functions - where forced truncation is the whole point.

cde/configure.ac

@@ -77,8 +77,6 @@ case "${host_os}" in
                 OSMINORVERSION=15
                 SOURCE_CPP_DEFINES="${SOURCE_CPP_DEFINES} -D_POSIX_SOURCE \
 -D_DEFAULT_SOURCE -D_BSD_SOURCE -D_SVID_SOURCE"
-                CPP_COMPILER_FLAGS="-fno-strict-aliasing -Wno-write-strings \
--Wno-unused-result"
                  ;;
 	freebsd*)
 		build_freebsd=yes
@@ -306,6 +304,20 @@ AC_PROG_CC_C99
 AC_PATH_X
 AC_PATH_XTRA
 
+dnl check some compiler flags
+C_FLAG_CHECK([-Wno-format-truncation])
+CXX_FLAG_CHECK([-Wno-format-truncation])
+
+C_FLAG_CHECK([-fno-strict-aliasing])
+CXX_FLAG_CHECK([-fno-strict-aliasing])
+
+C_FLAG_CHECK([-Wno-write-strings])
+CXX_FLAG_CHECK([-Wno-write-strings])
+
+C_FLAG_CHECK([-Wno-unused-result])
+CXX_FLAG_CHECK([-Wno-unused-result])
+
+
 dnl Add X11 goodies here
 EXTRA_LIBS="${X_LIBS} ${EXTRA_LIBS}"
 EXTRA_INCS="${X_CFLAGS} ${EXTRA_INCS}"