--- a/configure.in
+++ b/configure.in
@@ -21,7 +21,7 @@
 ### along with Octave; see the file COPYING.  If not, write to the Free
 ### Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
 
-AC_REVISION($Revision: 1.90 $)
+AC_REVISION($Revision: 1.91 $)
 AC_PREREQ(2.0)
 AC_INIT(src/octave.cc)
 AC_CONFIG_HEADER(config.h kpathsea/c-auto.h)
@@ -205,11 +205,13 @@
 changequote(,)dnl
   2.[6789].*|2.[123456789][0123456789].*)
 changequote([,])dnl
-    CXXFLAGS="-g -O"
-    AC_MSG_RESULT([defining CXXFLAGS to be $CXXFLAGS])
+    if test -z "$CXXFLAGS"; then
+      CXXFLAGS="-g -O"
+      AC_MSG_RESULT([defining CXXFLAGS to be $CXXFLAGS])
+      ADD_CXX_WALL=true
+    fi
     NO_IMPLICIT_TEMPLATES="-fno-implicit-templates"
     AC_MSG_RESULT([defining NO_IMPLICIT_TEMPLATES to be $NO_IMPLICIT_TEMPLATES])
-    ADD_CXX_WALL=true
   ;;
 changequote(,)dnl
   1.*|2.[012345].*)
@@ -257,11 +259,15 @@
 gcc_version=`$CC -v 2>&1 | grep "^gcc version" | sed 's/^gcc version *//'`
 case "$gcc_version" in
   2.*)
-    CFLAGS="-g -O"
-    AC_MSG_RESULT([defining CFLAGS to be $CFLAGS])
-    LDFLAGS="-g"
-    AC_MSG_RESULT([defining LDFLAGS to be $LDFLAGS])
-    ADD_CC_WALL=true
+    if test -z "$CFLAGS"; then
+      CFLAGS="-g -O"
+      AC_MSG_RESULT([defining CFLAGS to be $CFLAGS])
+      ADD_CC_WALL=true
+    fi
+    if test -z "$LDFLAGS"; then
+      LDFLAGS="-g"
+      AC_MSG_RESULT([defining LDFLAGS to be $LDFLAGS])
+    fi
   ;;
   1.*)
     AC_MSG_WARN([gcc version $gcc_version is likely to cause problems])
@@ -365,7 +371,6 @@
 if $use_f2c; then
   true
 else
-  F77=
   AC_CHECK_PROGS(F77, xlf cf77 f77 g77 fc)
 
   # If it looks like we have a Fortran compiler, try to determine
@@ -396,9 +401,15 @@
   AC_CHECK_PROG(F2C, f2c, f2c, [])
 fi
 
-F2CFLAGS=
-FFLAGS="-O"
-AC_MSG_RESULT([defining FFLAGS to be $FFLAGS])
+if test -z "$F2CFLAGS"; then
+  F2CFLAGS=
+fi
+
+if test -z "$FFLAGS"; then
+  FFLAGS="-O"
+  AC_MSG_RESULT([defining FFLAGS to be $FFLAGS])
+fi
+
 if test -n "$F77"; then
   export F77
   FLIBS=`sh $srcdir/flibs.sh`