manywarnings: update C warnings for GCC 14 Adjust for C programs compiled by GCC 14. (A C++ expert still needs to look at manywarnings-c++.m4.) * build-aux/gcc-warning.spec: Add warnings introduced in GCC 13. * m4/manywarnings.m4 (gl_MANYWARN_ALL_GCC): Add -Wflex-array-member-not-at-end, -Wmissing-variable-declarations.