Handle conflicting macros for FreeBSD

[jbloggz]

The unittest_proto3_bad_macros.proto test is failing on FreeBSD due to the GID_MAX/UID_MAX macros. This is because port_def.inc is missing these entries (as well as TRUE/FALSE) for FreeBSD. This PR adds those missing macros.
See compile errors below:

[ 87%] Building CXX object CMakeFiles/tests.dir/src/google/protobuf/unittest_preserve_unknown_enum2.pb.cc.o
[ 87%] Building CXX object CMakeFiles/tests.dir/src/google/protobuf/unittest_proto3.pb.cc.o
[ 87%] Building CXX object CMakeFiles/tests.dir/src/google/protobuf/unittest_proto3_arena.pb.cc.o
[ 87%] Building CXX object CMakeFiles/tests.dir/src/google/protobuf/unittest_proto3_arena_lite.pb.cc.o
[ 88%] Building CXX object CMakeFiles/tests.dir/src/google/protobuf/unittest_proto3_bad_macros.pb.cc.o
In file included from /usr/home/jpb/protobuf/src/google/protobuf/unittest_proto3_bad_macros.pb.cc:4:
/usr/home/jpb/protobuf/src/google/protobuf/unittest_proto3_bad_macros.pb.h:71:15: error: expected unqualified-id
constexpr GID GID_MAX = static_cast<GID>(0);
              ^
/usr/include/sys/limits.h:85:18: note: expanded from macro 'GID_MAX'
#define GID_MAX         UINT_MAX        /* max value for a gid_t */
                        ^
/usr/include/sys/limits.h:59:18: note: expanded from macro 'UINT_MAX'
#define UINT_MAX        __UINT_MAX      /* max value for an unsigned int */
                        ^
/usr/include/x86/_limits.h:58:20: note: expanded from macro '__UINT_MAX'
#define __UINT_MAX      0xffffffff      /* max value for an unsigned int */
                        ^
In file included from /usr/home/jpb/protobuf/src/google/protobuf/unittest_proto3_bad_macros.pb.cc:4:
/usr/home/jpb/protobuf/src/google/protobuf/unittest_proto3_bad_macros.pb.h:102:15: error: expected unqualified-id
constexpr UID UID_MAX = static_cast<UID>(0);
              ^
/usr/include/sys/limits.h:86:18: note: expanded from macro 'UID_MAX'
#define UID_MAX         UINT_MAX        /* max value for a uid_t */
                        ^
/usr/include/sys/limits.h:59:18: note: expanded from macro 'UINT_MAX'
#define UINT_MAX        __UINT_MAX      /* max value for an unsigned int */
                        ^
/usr/include/x86/_limits.h:58:20: note: expanded from macro '__UINT_MAX'
#define __UINT_MAX      0xffffffff      /* max value for an unsigned int */
                        ^
2 errors generated.
gmake[2]: *** [CMakeFiles/tests.dir/build.make:2038: CMakeFiles/tests.dir/src/google/protobuf/unittest_proto3_bad_macros.pb.cc.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:1178: CMakeFiles/tests.dir/all] Error 2
gmake: *** [Makefile:146: all] Error 2

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