diff --git a/configure.in b/configure.in
index 1d39379d8..c1722f6be 100644
--- a/configure.in
+++ b/configure.in
@@ -1,7 +1,7 @@
 dnl# -*- sh -*-
 dnl# the "configure" script is made from this by running GNU "autoconf"
 dnl#
-dnl# "$Id: configure.in,v 1.19 1999/01/31 07:43:08 bill Exp $"
+dnl# "$Id: configure.in,v 1.20 1999/02/01 19:40:25 mike Exp $"
 dnl#
 dnl# Configuration script for the Fast Light Tool Kit (FLTK).
 dnl#
@@ -36,11 +36,16 @@ else
   LIBCOMMAND="ar crs"
 fi
 
-DEBUGFLAG=""
+dnl# Clear debugging flags and only enable debugging if the user asks for
+dns# it.
 
-AC_ARG_ENABLE(debug, [  --enable-debug          turn on debugging [default=no]],if eval "test x$enable_debug = xyes"; then
+DEBUGFLAG=""
+CFLAGS="${CFLAGS:=}"
+CXXFLAGS="${CXXFLAGS:=}"
+
+AC_ARG_ENABLE(debug, [  --enable-debug          turn on debugging [default=no]],[if eval "test x$enable_debug = xyes"; then
   DEBUGFLAG="-g "
-fi)
+fi])
 AC_ARG_ENABLE(shared, [  --enable-shared         turn on shared libraries [default=no]],[
 if eval "test x$enable_shared = xyes"; then
   RANLIB=":"
@@ -138,25 +143,59 @@ dnl# do this last so messing with switches does not break tests
 if test -n "$GXX"; then
     CFLAGS="-Wall $CFLAGS"
     CXXFLAGS="-Wall -Wno-return-type $CXXFLAGS"
+    if test -z "$DEBUGFLAG"; then
+        CFLAGS="-O3 $CFLAGS"
+        CXXFLAGS="-O3 $CXXFLAGS"
+    fi
 else
-if test "`(uname) 2>/dev/null`" = IRIX; then
-  if expr "`(uname -r)`" \>= 6.2; then
-dnl# Uncomment these if you don't have IDO 7.2 or higher
-dnl#     CXX="CC -n32"
-dnl#     CC="cc -n32"
-dnl#     LD="ld -n32"
-dnl#     MAKEDEPEND="CC -M"
-    # -woff 3322 is necessary due to errors in Xlib headers on IRIX
-    CFLAGS="-O2 -fullwarn $CFLAGS"
-    CXXFLAGS="-O2 -fullwarn -woff 1685 -woff 3322 $CXXFLAGS"
-  else
-    CXXFLAGS="+w +pp $CXXFLAGS"
-  fi
-fi
+    case `(uname) 2>/dev/null` in
+        IRIX*)
+	    # Running some flavor of IRIX; see which version and
+	    # set things up according...
+	    if expr "`(uname -r)`" \>= 6.2; then
+	        # We are running IRIX 6.2 or higher; uncomment the following
+		# lines if you don't have IDO 7.2 or higher:
+		#
+		#     CXX="CC -n32"
+		#     CC="cc -n32"
+		#     LD="ld -n32"
+		#     MAKEDEPEND="CC -M"
+
+        	# -woff 3322 is necessary due to errors in Xlib headers on IRIX
+        	CFLAGS="-fullwarn $CFLAGS"
+        	CXXFLAGS="-fullwarn -woff 1685 -woff 3322 $CXXFLAGS"
+		if test -z "$DEBUGFLAG"; then
+        	    CFLAGS="-O3 $CFLAGS"
+        	    CXXFLAGS="-O3 $CXXFLAGS"
+		fi
+	    else
+        	CXXFLAGS="+w +pp $CXXFLAGS"
+		if test -z "$DEBUGFLAG"; then
+        	    CFLAGS="-O2 $CFLAGS"
+        	    CXXFLAGS="-O2 $CXXFLAGS"
+		fi
+	    fi
+	    ;;
+	HP-UX)
+	    # Running HP-UX; these options should work for the HP compilers.
+	    if test -z "$DEBUGFLAG"; then
+		CFLAGS="+O2 $CFLAGS"
+		CXXFLAGS="+O2 $CXXFLAGS"
+	    fi
+	    ;;
+	*)
+	    # Running some other operating system; inform the user they
+	    # should contribute the necessary options to fltk-bugs@easysw.com...
+	    echo "Building FLTK with default compiler optimizations; contact"
+	    echo "fltk-bugs@easysw.com with uname and compiler options needed"
+	    echo "for your platform, or set the CFLAGS and CXXFLAGS options"
+	    echo "before running configure."
+	    ;;
+    esac
 fi
 
-CFLAGS="$DEBUGFLAG$CFLAGS"
-CXXFLAGS="$DEBUGFLAG$CXXFLAGS"
+CFLAGS="$DEBUGFLAG $CFLAGS"
+CXXFLAGS="$DEBUGFLAG $CXXFLAGS"
 
 AC_SUBST(LIBNAME)
 AC_SUBST(LIBCOMMAND)
@@ -165,5 +204,5 @@ AC_CONFIG_HEADER(config.h:configh.in)
 AC_OUTPUT(makeinclude)
 
 dnl#
-dnl# End of "$Id: configure.in,v 1.19 1999/01/31 07:43:08 bill Exp $".
+dnl# End of "$Id: configure.in,v 1.20 1999/02/01 19:40:25 mike Exp $".
 dnl#