glibc has two versions of strerror_r, a standards compliant one and a GNU

specific one. The GNU one is chosen when _GNU_SOURCE is defined. g++ always
defines _GNU_SOURCE on linux platforms because glibc's headers won't compile
in C++ mode without it. The GNU strerror_r doesn't always modify the buffer
which causes empty error messages on linux.

This patch changes MakeErrMsg to use the return value of strerror_r to get
the string instead of assuming the buffer will be modified, on GLIBC.

Patch by Benjamin Kramer!

llvm-svn: 73396