compat/fsmonitor/fsm-listen-darwin: add MacOS header files for FSEvent

compat/fsmonitor/fsm-listen-darwin: add MacOS header files for FSEvent

Include MacOS system declarations to allow us to use FSEvent and
CoreFoundation APIs.  We need different versions of the declarations
for GCC vs. clang because of compiler and header file conflicts.

While it is quite possible to #include Apple's CoreServices.h when
compiling C source code with clang, trying to build it with GCC
currently fails with this error:

In file included
   from /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/System/...
   ...Library/Frameworks/Security.framework/Headers/AuthSession.h:32,
   from /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/System/...
   ...Library/Frameworks/Security.framework/Headers/Security.h:42,
   from /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/System/...
   ...Library/Frameworks/CoreServices.framework/Frameworks/...
   ...OSServices.framework/Headers/CSIdentity.h:43,
   from /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/System/...
   ...Library/Frameworks/CoreServices.framework/Frameworks/...
   ...OSServices.framework/Headers/OSServices.h:29,
   from /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/System/...
   ...Library/Frameworks/CoreServices.framework/Frameworks/...
   ...LaunchServices.framework/Headers/IconsCore.h:23,
   from /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/System/...
   ...Library/Frameworks/CoreServices.framework/Frameworks/...
   ...LaunchServices.framework/Headers/LaunchServices.h:23,
   from /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/System/...
   ...Library/Frameworks/CoreServices.framework/Headers/CoreServices.h:45,

     /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/System/...
     ...Library/Frameworks/Security.framework/Headers/Authorization.h:193:7:
     error: variably modified 'bytes' at file scope
       193 | char bytes[kAuthorizationExternalFormLength];
           |      ^~~~~

The underlying reason is that GCC (rightfully) objects that an `enum`
value such as `kAuthorizationExternalFormLength` is not a constant
(because it is not, the preprocessor has no knowledge of it, only the
actual C compiler does) and can therefore not be used to define the size
of a C array.

This is a known problem and tracked in GCC's bug tracker:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93082

In the meantime, let's not block things and go the slightly ugly route
of declaring/defining the FSEvents constants, data structures and
functions that we need, so that we can avoid above-mentioned issue.

Let's do this _only_ for GCC, though, so that the CI/PR builds (which
build both with clang and with GCC) can guarantee that we _are_ using
the correct data types.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>