Support for C11's _Thread_local was introduced in GCC in the 4.9 series,
even though we make the C11 compiler flags available in CMake with GCC
>= 4.6.
FreeBSD's runetype.h uses _Thread_local, which causes CMake's own build
to fail when using GCC < 4.9 and -std=gnu11:
/usr/include/runetype.h:92:22: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'const'
extern _Thread_local const _RuneLocale *_ThreadRuneLocale;
Add a test for _Thread_local support and only build CMake itself with
C11 support if it works.
Bug: http://www.cmake.org/Bug/view.php?id=15741
ffa6f057b4