Respect CFLAGS env var and support MacPorts and BSD include directories

Users should be able to pass custom compilation flags such as include
directories via CFLAGS env var.

It would be convenient for MacPorts users if libraries installed in /opt/local
worked by default. (/opt/local/lib is supported since cffi#139.)

/usr/local/include/ was added for OpenBSD because cffi finds it useful in
cc07246

Fixes orivej/pzmq#18

toolchain/c-toolchain.lisp

@@ -208,13 +208,8 @@
           *cc-flags*
           (append
            arch-flags
-           ;; For MacPorts
-           #+darwin (list "-I" "/opt/local/include/")
            ;; ECL internal flags
-           #+ecl (parse-command-flags c::*cc-flags*)
-           ;; FreeBSD non-base header files
-           #+freebsd (list "-I" "/usr/local/include/")
-           (split-cflags (getenv "CFLAGS")))
+           #+ecl (parse-command-flags c::*cc-flags*))
           *ld* *cc*
           *ld-exe-flags* `(,@arch-flags #-darwin "-Wl,--export-dynamic")
           *ld-dll-flags* (list* #+darwin "-dynamiclib" ;; -bundle ?
@@ -223,12 +218,23 @@
           *linkkit-start* nil
           *linkkit-end* nil)))
 
+(defun common-toolchain-parameters ()
+  (setf *cc-flags*
+        (append
+         *cc-flags*
+         ;; For MacPorts
+         #+darwin (list "-I" "/opt/local/include/")
+         (split-cflags (getenv "CFLAGS"))
+         ;; FreeBSD non-base header files
+         #+(or freebsd openbsd) (list "-I" "/usr/local/include/"))))
+
 (defun ensure-toolchain-parameters ()
   #+clisp (unless *cc* (clisp-toolchain-parameters))
   #+ecl (unless *cc* (ecl-toolchain-parameters))
   #+mkcl (unless *cc* (mkcl-toolchain-parameters))
   #+sbcl (unless *cc* (sbcl-toolchain-parameters))
-  (unless *cc* (default-toolchain-parameters)))
+  (unless *cc* (default-toolchain-parameters))
+  (common-toolchain-parameters))
 
 ;; Actually initialize toolchain parameters
 (ignore-errors (ensure-toolchain-parameters))