Add Mac gcc6 build configuration (gcc via brew)

config/Makefile.darwin-gcc6

+# For building with Brew gcc:
+# $ brew install gcc
+GLEW_DEST = /usr/local
+NAME = $(GLEW_NAME)
+CC = gcc-6
+LD = gcc-6
+CFLAGS.EXTRA = -dynamic -fno-common
+CFLAGS.EXTRA += -std=c89
+# Lots of warnings with -pedantic
+# warning: ISO C forbids conversion of object pointer to function pointer type
+#CFLAGS.EXTRA += -pedantic
+#CFLAGS.EXTRA += -no-cpp-precomp
+LDFLAGS.EXTRA =
+ifneq (undefined, $(origin GLEW_APPLE_GLX))
+CFLAGS.EXTRA += -I/usr/X11R6/include -D'GLEW_APPLE_GLX'
+LDFLAGS.GL = -L/usr/X11R6/lib -lGL -lX11
+else
+LDFLAGS.GL = -framework OpenGL
+endif
+LDFLAGS.STATIC =
+LDFLAGS.DYNAMIC =
+WARN = -Wall -W
+POPT = -Os
+CFLAGS.EXTRA += -fPIC
+BIN.SUFFIX =
+LIB.SONAME    = lib$(NAME).$(SO_MAJOR).dylib
+LIB.DEVLNK    = lib$(NAME).dylib
+LIB.SHARED    = lib$(NAME).$(SO_VERSION).dylib
+LIB.STATIC    = lib$(NAME).a
+LDFLAGS.SO    = -dynamiclib -install_name $(GLEW_DEST)/lib/$(LIB.SHARED) -current_version $(SO_VERSION) -compatibility_version $(SO_MAJOR)