[SV 65537] Update to newer gnulib

* bootstrap.conf: Switch to gnulib stable-202401 branch.
* maintMakefile: Support an EXTRA_CFLAGS variable in maintainer mode.
* README.git: Describe how to use it.

README.git

@@ -155,7 +155,8 @@ NOTE! This method builds GNU Make in "maintainer mode".  Make programs built
       If you want to build from Git with "maintainer mode" disabled, add
       "MAKE_MAINTAINER_MODE=" to the make command line.  If you want to turn
       off the extra warning flags, add "MAKE_CFLAGS=" to the make command
-      line.
+      line.  If you want to keep the warnings but not fail, add
+      "EXTRA_CFLAGS=-Wno-error" to the make command line.
 
       For example:
         $ ./configure

bootstrap.conf

@@ -19,7 +19,7 @@
 checkout_only_file=README.git
 
 # Choose a specific version of gnulib, when checking out
-GNULIB_REVISION=stable-202307
+GNULIB_REVISION=stable-202401
 
 # Always copy files rather than symlink
 copy=true

maintMakefile

@@ -34,7 +34,10 @@ MAKE_CFLAGS := -C -Wall -Wextra -Werror -Wwrite-strings -Wshadow \
 	-Wtype-limits -Wunused-but-set-parameter -Wlogical-op -Wpointer-arith \
 	-Wignored-qualifiers -Wformat-signedness -Wduplicated-cond
 
-AM_CFLAGS += $(MAKE_CFLAGS)
+# Allow extra options without overriding MAKE_CFLAGS
+EXTRA_CFLAGS :=
+
+AM_CFLAGS += $(MAKE_CFLAGS) $(EXTRA_CFLAGS)
 
 # Unfortunately the Guile headers are sometimes broken.  Convince GCC
 # to treat them as system headers so warnings are ignored.