Makefile: Fix build on MSYS2 and Cygwin

The host compiler on MSYS2 and Cygwin does not allow the -fPIC option,
issuing a warning that is treated as an error and stops the build.
Detect whether we're running under MSYS2 or Cygwin and avoid adding
-fPIC to prevent the error from happening.

Tested on Linux, MSYS2 and Cygwin.

Signed-off-by: Carles Cufi <carles.cufi@gmail.com>
[dwg: Added explicit empty CFLAGS for clarity]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>