[benchmark] Fix win32 link on case-sensitive fs

Summary: This fixes cross-builds with MinGW from case-sensitive file-systems (on Linux)

This is a cherry-pick from
https://github.com/google/benchmark/pull/840
https://github.com/google/benchmark/commit/8e48105d465c586068dd8e248fe75a8971c6ba3a

Original patch by: @jschueller (Julien Schueller) !

Differential Revision: https://reviews.llvm.org/D61371

llvm-svn: 367356