Fix issue with CMake Xcode generator (#1266)

There is a known issue with CMake and its Xcode generator when
the target has no sources of its own -- see

https://gitlab.kitware.com/cmake/cmake/-/issues/23688

A workaround is to define an empty source file that can be used
to trigger the build path for the framework. Without this hack,
the framework is missing the shared library component.

With this change, the following now runs successfully:

``
% cmake -G Xcode -Denable-sdl2=NO -Denable-readline=NO -Denable-dbus=NO -DCMAKE_INSTALL_PREFIX=$PWD ..
% xcodebuild
```

src/CMakeLists.txt

@@ -205,7 +205,7 @@ set ( public_main_HEADER
     ${FluidSynth_BINARY_DIR}/include/fluidsynth.h
 )
 
-configure_file ( ${FluidSynth_SOURCE_DIR}/include/fluidsynth/version.h.in 
+configure_file ( ${FluidSynth_SOURCE_DIR}/include/fluidsynth/version.h.in
                  ${FluidSynth_BINARY_DIR}/include/fluidsynth/version.h )
 configure_file ( ${FluidSynth_SOURCE_DIR}/include/fluidsynth.cmake
                  ${public_main_HEADER} )
@@ -275,6 +275,14 @@ if ( LIBFLUID_CPPFLAGS )
     PROPERTIES COMPILE_FLAGS ${LIBFLUID_CPPFLAGS} )
 endif ( LIBFLUID_CPPFLAGS )
 
+# The CMake Xcode generator fails to build the framework unless it references
+# a source file. Create an empty one in the build directory to use for this
+# purpose.
+if ( MACOSX_FRAMEWORK )
+  set ( MACOSX_FRAMEWORK_SOURCE_HACK ${CMAKE_BINARY_DIR}/empty_file_for_linking_issue.cpp )
+  file ( TOUCH ${MACOSX_FRAMEWORK_SOURCE_HACK} )
+endif ( MACOSX_FRAMEWORK )
+
 # Note: by default this target creates a shared object (or dll). To build a
 # static library instead, set the option BUILD_SHARED_LIBS to FALSE.
 # Further note: The headers must be explicitly added here to have CMake install
@@ -283,6 +291,7 @@ add_library ( libfluidsynth
     $<TARGET_OBJECTS:libfluidsynth-OBJ>
     ${public_main_HEADER}
     ${public_HEADERS}
+    ${MACOSX_FRAMEWORK_SOURCE_HACK}
 )
 
 if ( MACOSX_FRAMEWORK )