added check for compiler support of Wextra-semi

Added check to configure.ac to test if the compiler supports Wextra-semi. If the test passes the flag is added to CFLAGS, if not it is omitted. Added a macro to carry out the test (source: https://github.com/autoconf-archive/autoconf-archive/commit/0e7f3286a04ad41f6fbf5247fef5401fda49ca5a / https://www.gnu.org/software/autoconf-archive/ax_check_compile_flag.html) to the m4 directory. Fixes #40119 (closed)

When I last did a merge request to solve this issue I also had commits in that were not relevant to this issue. I have rectified that this time.