makefile.m32: allow to override gcc, ar and ranlib
makefile.m32: allow to override gcc, ar and ranlib

Allow to ovverride certain build tools, making it possible to
use LLVM/Clang to build libssh2. The default behavior is unchanged.
To build with clang (as offered by MSYS2), these settings can
be used:

LIBSSH2_CC=clang
LIBSSH2_AR=llvm-ar
LIBSSH2_RANLIB=llvm-ranlib

Also adjust ranlib parameters to be compatible with LLVM/Clang's
ranlib tool.

Closes https://github.com/libssh2/libssh2/pull/214

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2