configure: Added Guobao Shen's comments on using MacPorts GCC

configure/os/CONFIG_SITE.Common.darwin-x86

@@ -7,17 +7,31 @@
 #-------------------------------------------------------
 
 # Select which CPU architecture(s) to include in your MacOS binaries:
-#   i386
-#   x86_64  - Needs MacOS 10.4 with the Universal SDK, or 10.5 and later
+#   i386, x86_64, or both (fat binaries).
 
 #ARCH_CLASS = i386
 ARCH_CLASS = x86_64
 #ARCH_CLASS = i386 x86_64
 
 #
-# Comment out the followings 3 lines to build with GCC instead of CLANG.
+# Comment out the following 3 lines to build with Apple's GCC instead of CLANG.
 #
 CC  = clang
 CCC = clang++
 GNU = NO
 
+
+# To use MacPorts GCC, comment out ALL the settings above this line,
+# then uncomment (and modify if necessary) the following instead:
+
+#GNU_DIR = /opt/local
+#CC = $(GNU_BIN)/gcc -m64
+#CCC = $(GNU_BIN)/g++ -m64
+
+# If you see this or similar errors while building in the src/cap5 directory
+#   gcc: error: unrecognized option '-no-cpp-precomp'
+# the problem is due to the ccflags configuration that your version of Perl
+# was built with.  You can replace the Cap5_CFLAGS setting in the Makefile
+# with a hand-edited set of flags for building that Perl library, or ignore
+# this problem if you don't need to use Channel Access from Perl.
+