Make compilable on Cygwin

This patch makes FF compilable on Cygwin. HEAD as of Aug 04 11:30JST (8792171e8c805692c3815e10101c1ff3c74c2f9e) doesn't compile, complaining:
```
gdraw.c: In function 'MacServiceReadFDs':
gdraw.c:1063:20: error: storage size of 'timeout' isn't known
gdraw.c:1076:2: error: implicit declaration of function 'select' [-Werror=implicit-function-declaration]
gdraw.c:1063:20: warning: unused variable 'timeout' [-Wunused-variable]
cc1: some warnings being treated as errors
Makefile:771: recipe for target `libgdraw_la-gdraw.lo' failed
make[2]: *** [libgdraw_la-gdraw.lo] Error 1
make[2]: ディレクトリ `/home/TT/git/fontforge/gdraw' から出ます
Makefile:785: recipe for target `all-recursive' failed
make[1]: *** [all-recursive] Error 1
make[1]: ディレクトリ `/home/TT/git/fontforge' から出ます
Makefile:635: recipe for target `all' failed
make: *** [all] Error 2
```

MinGW-specific codes should also be applied for Cygwin build. While `MacServiceReadFDs()` turns empty (i.e. does nothing) on MinGW, Cygwin's compiler attempts to use the _unknown_ `struct timeval`. Cygwin also tries to read the _unavailable_ `execinfo.h` for backtrace.