CheckTypeSize: Fix with clang '-Werror,-Wmissing-variable-declarations'
Resolves issue building with missing variable declarations, error:
/src/cmake_clang/CMakeFiles/CheckTypeSize/CMAKE_SIZEOF_UNSIGNED_LONG.c:24:6: error: no previous extern declaration for non-static variable 'info_size' [-Werror,-Wmissing-variable-declarations]
char info_size[] = {'I', 'N', 'F', 'O', ':', 's','i','z','e','[',
^
1 error generated.
Topic-rename: CheckTypeSize-decl-warning