CxxUtils: Fix glibc test.
Use __has_include when looking for the system features.h. Removes need for platform-dependent #ifdef.
Add dummy version of __GLIBC_PREREQ if it's not defined, so that we can use it without an additional test.
Use __has_include when looking for the system features.h. Removes need for platform-dependent #ifdef.
Add dummy version of __GLIBC_PREREQ if it's not defined, so that we can use it without an additional test.