[libc++] Workaround the absence of the __GLIBC_USE macro in glibc versions prior to 2.25.

This change correct a configuration check that relies on the glibc __GLIBC_USE
macro being defined. Previously, the function-like macro was expanded without
ensuring it was actually defined. This resulted in compilation failures for
glibc versions prior to 2.25 (the glibc version in which the macro was added).

Differential Revision: https://reviews.llvm.org/D130946