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

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!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@73396 91177308-0d34-0410-b5e6-96231b3b80d8