Bump minimum GCC version to 13

Among other things, gets rid of issue
"C++ preprocessor ignores #pragma GCC diagnostic":
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53431

Change-Id: I37492d49d3671e2f0df5e36f727690f651d4fa9e
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/202293
Tested-by: Ilmari Lauhakangas <ilmari.lauhakangas@libreoffice.org>
Reviewed-by: Ilmari Lauhakangas <ilmari.lauhakangas@libreoffice.org>

README.md

@@ -42,7 +42,7 @@ run and compile LibreOffice, also used by the TDF builds:
     * Build: 13 or later + Xcode 14.3 or later (using latest version available for a given version of macOS)
 * Linux:
     * Runtime: RHEL 9 or CentOS 9 and comparable
-    * Build: either GCC 12; or Clang 18 with libstdc++ 11
+    * Build: either GCC 13; or Clang 18 with libstdc++ 11
 * iOS (only for LibreOfficeKit):
     * Runtime: 14.5 (only support for newer i devices == 64 bit)
     * Build: Xcode 12.5 and iPhone SDK 14.5

configure.ac

@@ -4972,8 +4972,8 @@ if test "$GCC" = "yes" -a -z "$COM_IS_CLANG"; then
 
     AC_MSG_RESULT([gcc $_gcc_version ($gcc_full_version)])
 
-    if test "$gcc_full_version" -lt 120000; then
-        AC_MSG_ERROR([GCC $_gcc_version is too old, must be at least GCC 12])
+    if test "$gcc_full_version" -lt 130000; then
+        AC_MSG_ERROR([GCC $_gcc_version is too old, must be at least GCC 13])
     fi
 else
     # Explicitly force GCC_VERSION to be empty, even for Clang, to check incorrect uses.