http://lists.gnu.org/archive/html/bug-ed/2008-12/msg00001.html

2007-04-16  Mike Frysinger  <vapier@gentoo.org>

	* Do not set CC/CXX/CPPFLAGS/LDFLAGS to "" so that user can
	  override if they so choose.
	* Only set CFLAGS/CXXFLAGS if user did not specify any.

--- ed-1.0/configure
+++ ed-1.0/configure
@@ -24,12 +37,6 @@
 mandir='$(datadir)/man'
 sysconfdir='$(prefix)/etc'
 program_prefix=
-CC=
-CXX=
-CPPFLAGS=
-CFLAGS='-Wall -W -O2'
-CXXFLAGS='-Wall -W -O2'
-LDFLAGS=
 
 # Loop over all args
 while [ x"$1" != x ] ; do
@@ -106,6 +113,14 @@
 	esac
 done
 
+# Defaults if the user did not select any
+if [ x"${CFLAGS+set}" != xset ] ; then
+	CFLAGS='-Wall -W -O2'
+fi
+if [ x"${CXXFLAGS+set}" != xset ] ; then
+	CXXFLAGS='-Wall -W -O2'
+fi
+
 # Find the source files, if location was not specified.
 srcdirtext=
 if [ x"${srcdir}" = x ] ; then