mingw/cygwin: Stop adding -fno-common to gcc CFLAGS

This was done to work around toolchain bugs that have long been fixed.

configure

@@ -2796,7 +2796,6 @@ case $target_os in
         dlltool="${cross_prefix}dlltool"
         ranlib=:
         enable dos_paths
-        check_cflags -fno-common
         add_cppflags -U__STRICT_ANSI__
         ;;
     cygwin*)
@@ -2812,7 +2811,6 @@ case $target_os in
         SHFLAGS='-shared -Wl,--out-implib,$(SUBDIR)lib$(FULLNAME).dll.a'
         objformat="win32"
         enable dos_paths
-        check_cflags -fno-common
         add_cppflags -U__STRICT_ANSI__
         ;;
     *-dos|freedos|opendos)