improve error when /usr/include isn't found [PR90835]

This is a pretty simple patch that ought to help Darwin users understand
better why their build is failing when they forget to pass the
--with-sysroot= flag to configure.

gcc/ChangeLog:

	PR target/90835
	* Makefile.in: improve error message when /usr/include is
	missing

gcc/Makefile.in

@@ -55,6 +55,7 @@ MAKEOVERRIDES =
 # -------------------------------
 
 build=@build@
+build_os=@build_os@
 host=@host@
 host_noncanonical=@host_noncanonical@
 host_os=@host_os@
@@ -3333,8 +3334,13 @@ stmp-fixinc: gsyslimits.h macro_list fixinc_list \
 	    multi_dir=`echo $${ml} | sed -e 's/^[^;]*;//'`; \
 	    fix_dir=include-fixed$${multi_dir}; \
 	    if ! $(inhibit_libc) && test ! -d ${BUILD_SYSTEM_HEADER_DIR}; then \
-	      echo The directory that should contain system headers does not exist: >&2 ; \
+	      echo "The directory (BUILD_SYSTEM_HEADER_DIR) that should contain system headers does not exist:" >&2 ; \
 	      echo "  ${BUILD_SYSTEM_HEADER_DIR}" >&2 ; \
+	      case ${build_os} in \
+	        darwin*) \
+	          echo "(on Darwin this usually means you need to pass the --with-sysroot= flag to point to a valid MacOS SDK)" >&2; \
+	          ;; \
+	      esac; \
 	      tooldir_sysinc=`echo "${gcc_tooldir}/sys-include" | sed -e :a -e "s,[^/]*/\.\.\/,," -e ta`; \
 	      if test "x${BUILD_SYSTEM_HEADER_DIR}" = "x$${tooldir_sysinc}"; \
 	      then sleep 1; else exit 1; fi; \