build: make autotools build system work with mingw
build: remove mingw makefile

Commit e52aa6c adds MinGW support to the autotools build.  Remove the
Makefile, it's no longer needed.
build: remove _GNU_SOURCE macro definition

Compile libuv without -D_GNU_SOURCE, remove mention from the README.

The only place where it's still used is in the test suite and only
because test/test-fs.c uses struct stat directly.