Compile with cc rather than gcc whenever possible

Makefile.system

@@ -17,16 +17,20 @@ NETLIB_LAPACK_DIR = $(TOPDIR)/lapack-netlib
 #   http://stackoverflow.com/questions/4029274/mingw-and-make-variables
 # - Default value is 'cc' which is not always a valid command (e.g. MinGW).
 ifeq ($(origin CC),default)
+
+# Check if $(CC) refers to a valid command and set the value to gcc if not
+ifneq ($(findstring cmd.exe,$(SHELL)),)
+ifeq ($(shell where $(CC) 2>NUL),)
+CC = gcc
+endif
+else # POSIX
+ifeq ($(shell command -v $(CC) 2>/dev/null),)
 CC = gcc
-# Change the default compile to clang on Mac OSX.
-# http://stackoverflow.com/questions/714100/os-detecting-makefile
-UNAME_S := $(shell uname -s)
-ifeq ($(UNAME_S),Darwin)
-     CC = clang
-#     EXTRALIB += -Wl,-no_compact_unwind
 endif
 endif
 
+endif # CC is set to default
+
 # Default Fortran compiler (FC) is selected by f_check.
 
 ifndef MAKEFILE_RULE