configure: test the metal compiler before use

Apparently Metal.framework is included with the command line tools
(and thus may be present without Xcode), but the Metal compiler is only
included as part of Xcode.

configure

@@ -6365,6 +6365,8 @@ enabled videotoolbox && {
     check_func_headers CoreVideo/CVImageBuffer.h kCVImageBufferTransferFunction_SMPTE_ST_428_1 "-framework CoreVideo"
 }
 
+enabled metal && test_cmd $metalcc -v || disable metal
+
 check_struct "sys/time.h sys/resource.h" "struct rusage" ru_maxrss
 
 check_type "windows.h dxva.h" "DXVA_PicParams_AV1" -DWINAPI_FAMILY=WINAPI_FAMILY_DESKTOP_APP -D_CRT_BUILD_DESKTOP_APP=0