--- 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_TRADITIONAL=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, [