On macOS, don't build native GUI if XCode is not installed.

MacOS has two way to install tools you need to develop (make, git, cc, ...).
Either install Xcode or something called Command Line Tools.
But the Command Line Tools version doesn't include all the necessary programs to build Apple applications.
For example xcodebuild, which if invoked without XCode installed will tell that xcode is not installed and
exit with an non 0 exit code.

src/Makefile.OCaml

@@ -72,7 +72,14 @@ endif
 
 .PHONY: macuimaybe
 ifeq ($(OSARCH), Darwin)
-  macuimaybe: macui
+  # If XCode is not installed, xcodebuild is just a placeholder telling that XCode is not installed
+  # and any invocation of xcodebuild results in a non 0 exit code.
+  ifeq ($(shell xcodebuild -version > /dev/null; echo $$?), 0)
+    macuimaybe: macui
+  else
+    macuimaybe:
+	$(info Not building macOS native GUI because XCode is not installed.)
+  endif
 else
   macuimaybe:
 	$(info Not on macOS. macOS native GUI will not be built.)