Fail if trying to build headless on MacOS.

AFAICT this is doomed to fail anyways, so failing early with an
informative error message is an improvement.

It should be possible to get it working though.

CMakeLists.txt

@@ -51,6 +51,12 @@ set(RUN_IN_PLACE ${DEFAULT_RUN_IN_PLACE} CACHE BOOL
 
 set(BUILD_CLIENT TRUE CACHE BOOL "Build client")
 set(BUILD_HEADLESS TRUE CACHE BOOL "Build in headless mode")
+
+if(APPLE AND BUILD_HEADLESS)
+ message(FATAL_ERROR "Headless mode is not supported on MacOS yet. It should be possible, PRs welcome.")
+endif()
+
+
 set(BUILD_SERVER FALSE CACHE BOOL "Build server")
 set(BUILD_UNITTESTS TRUE CACHE BOOL "Build unittests")
 set(BUILD_BENCHMARKS FALSE CACHE BOOL "Build benchmarks")