--- orig/configure.in
+++ mod/configure.in
@@ -3126,6 +3126,14 @@
 	CPP_NEED_TRADITIONAL=no,
 	CPP_NEED_TRADITIONAL=yes)
 
+## New-fangled C preprocessors consider `//' to start a one-line comment.
+## The -ansi option forces GCC's cpp to ignore this syntax.
+
+AC_EGREP_CPP(yes//yes,
+	[yes//yes],
+	CPP_NEED_ANSI=no,
+	CPP_NEED_ANSI=yes)
+
 AC_OUTPUT(Makefile lib-src/Makefile.c:lib-src/Makefile.in oldXMenu/Makefile \
 	man/Makefile lwlib/Makefile src/Makefile.c:src/Makefile.in \
 	lisp/Makefile lispref/Makefile lispintro/Makefile leim/Makefile, [
@@ -3160,6 +3168,13 @@
   CPPFLAGS="$CPPFLAGS -traditional"
 fi
 
+# Recent GNU cpp versions understand C++-style one-line comments
+# unless invoked with -ansi.  (Works with GCC 3.3, 3.4, 4.0)
+
+if test "x$GCC" = xyes && test "x$CPP_NEED_ANSI" = xyes; then
+  CPPFLAGS="$CPPFLAGS -ansi"
+fi
+
 echo creating lib-src/Makefile
 ( cd lib-src
   rm -f junk.c junk1.c junk2.c
@@ -3200,7 +3215,7 @@
 # This is how we know whether to re-run configure in certain cases.
 touch src/config.stamp
 
-], [GCC="$GCC" NON_GNU_CPP="$NON_GNU_CPP" CPP="$CPP" CPP_NEED_TRADITIONAL="$CPP_NEED_TRADITIONAL" CPPFLAGS="$CPPFLAGS"])
+], [GCC="$GCC" NON_GNU_CPP="$NON_GNU_CPP" CPP="$CPP" CPP_NEED_TRADITIONAL="$CPP_NEED_TRADITIONAL" CPP_NEED_ANSI="$CPP_NEED_ANSI" CPPFLAGS="$CPPFLAGS"])
 
 m4_if(dnl	Do not change this comment
    arch-tag: 156a4dd5-bddc-4d18-96ac-f37742cf6a5e