CONFIG: enable xcrun with cc/c++ based on WM_COMPILE_CONTROL (#2965)

- adding in +xcrun into WM_COMPILE_CONTROL changes the compiler
  settings as follows (for MacOS)

      cc         := xcrun cc
      CC         := xcrun c++ -std=c++14

etc/bashrc

@@ -91,6 +91,7 @@ export WM_COMPILE_OPTION=Opt
 #   +openmp : with openmp
 #   ~openmp : without openmp
 #   +ccache : use ccache
+#   +xcrun  : use xcrun and native compilers [MacOS]
 #   ccache=... : ccache command (unquoted, single/double or <> quoted)
 #   version=... : compiler suffix (eg, "11" for gcc-11)
 #export WM_COMPILE_CONTROL="+gold"

etc/cshrc

@@ -91,6 +91,7 @@ setenv WM_COMPILE_OPTION Opt
 #   +openmp : with openmp
 #   ~openmp : without openmp
 #   +ccache : use ccache
+#   +xcrun  : use xcrun and native compilers [MacOS]
 #   ccache=... : ccache command (unquoted, single/double or <> quoted)
 #   version=... : compiler suffix (eg, "11" for gcc-11)
 #setenv WM_COMPILE_CONTROL "+gold"

wmake/rules/darwin64Clang/c

@@ -1,6 +1,10 @@
 #------------------------------------------------------------------------------
 include $(GENERAL_RULES)/Clang/c
 
+ifneq (,$(findstring +xcrun,$(WM_COMPILE_CONTROL)))
+cc         := xcrun cc
+endif
+
 cARCH      := -m64 -ftrapping-math
 
 ifneq (,$(strip $(WM_COMPILE_OPTION)))

wmake/rules/darwin64Clang/c++

@@ -1,6 +1,10 @@
 #------------------------------------------------------------------------------
 include $(GENERAL_RULES)/Clang/c++
 
+ifneq (,$(findstring +xcrun,$(WM_COMPILE_CONTROL)))
+CC         := xcrun c++ -std=c++14
+endif
+
 c++ARCH    := -m64 -pthread -ftrapping-math
 
 ifneq (,$(strip $(WM_COMPILE_OPTION)))