cmake: support for building git on windows with mingw

cmake: support for building git on windows with mingw

This patch facilitates building git on Windows with CMake using MinGW

NOTE: The funtions unsetenv and hstrerror are not checked in Windows
builds.
Reasons
NO_UNSETENV is not compatible with Windows builds.
lines 262-264 compat/mingw.h

compat/mingw.h(line 25) provides a definition of hstrerror which
conflicts with the definition provided in
git-compat-util.h(lines 733-736).

To use CMake on Windows with MinGW do this:
cmake `relative-path-to-srcdir` -G "MinGW Makefiles"

Signed-off-by: Sibi Siddharthan <redacted>
Signed-off-by: Junio C Hamano <redacted>