buildsys: add maintainer mode

Maintainer mode (on by default) controls whether our build system
automatically regenerates `configure` if `configure.ac` or any of its other
sources changes, by running `autoconf`. Similarly, it regenerates
`src/config.h.in` if necessary by running `autoheader`.

While this is very useful when developing GAP, it can be problematic if a user
wants to compile a release version of GAP but does not have autoconf
installed; or if for some reason the GAP source code is readonly and they want
to run an out-of-tree build. Thus we allow turning this off.

GNUmakefile.in

@@ -27,6 +27,9 @@ GC_SOURCES = @GC_SOURCES@
 COMPAT_MODE = @COMPAT_MODE@
 GAPARCH = @GAPARCH@
 
+# maintainer mode
+MAINTAINER_MODE = @MAINTAINER_MODE@
+
 # GAP kernel version
 GAP_KERNEL_MINOR_VERSION = @gap_kernel_minor_version@
 GAP_KERNEL_MAJOR_VERSION = @gap_kernel_major_version@

Makefile.rules

@@ -1272,13 +1272,15 @@ configure_deps = $(srcdir)/configure.ac $(ACLOCAL_M4)
 config.status: $(srcdir)/configure
 	$(SHELL) ./config.status --recheck
 
+ifneq ($(MAINTAINER_MODE),no)
 $(srcdir)/configure: $(configure_deps)
 	@if command -v autoconf >/dev/null 2>&1 ; then \
 	   echo "running autoconf" ; \
 	   cd $(srcdir) && autoconf ; \
 	 else \
 	   echo "autoconf not available, proceeding with stale configure" ; \
 	 fi
+endif
 
 gen/config.h: gen/stamp-h
 	@if test ! -f $@; then rm -f gen/stamp-h; else :; fi
@@ -1289,6 +1291,7 @@ gen/stamp-h: $(srcdir)/src/config.h.in config.status
 	$(SHELL) ./config.status gen/config.h
 	echo > $@
 
+ifneq ($(MAINTAINER_MODE),no)
 $(srcdir)/src/config.h.in: $(configure_deps) 
 	@if command -v autoheader >/dev/null 2>&1 ; then \
 	   echo "running autoheader" ; \
@@ -1298,6 +1301,7 @@ $(srcdir)/src/config.h.in: $(configure_deps)
 	   echo "autoheader not available, proceeding with stale config.h" ; \
 	 fi
 	touch $@
+endif
 
 GNUmakefile: $(srcdir)/GNUmakefile.in config.status
 	$(SHELL) ./config.status $@

configure.ac

@@ -214,6 +214,27 @@ AC_ARG_ENABLE([debug],
 AC_MSG_CHECKING([whether to enable debug mode])
 AC_MSG_RESULT([$enable_debug])
 
+dnl
+dnl Maintainer mode (on by default) controls whether our build system
+dnl automatically regenerates `configure` if `configure.ac` or any of its
+dnl other sources changes, by running `autoconf`. Similarly, it regenerates
+dnl `src/config.h.in` if necessary by running `autoheader`.
+dnl
+dnl While this is very useful when developing GAP, it can be problematic if
+dnl a user wants to compile a release version of GAP but does not have
+dnl autoconf installed; or if for some reason the GAP source code is readonly
+dnl and they want to run an out-of-tree build. Thus we allow turning this
+dnl off.
+AC_ARG_ENABLE([maintainer-mode],
+    [AS_HELP_STRING([--disable-maintainer-mode], [disable maintainer mode])],
+    [],
+    [enable_maintainer_mode=yes]
+    )
+AC_MSG_CHECKING([whether to enable maintainer-mode mode])
+AC_MSG_RESULT([$enable_maintainer_mode])
+AC_SUBST([MAINTAINER_MODE], [$enable_maintainer_mode])
+
+dnl
 AC_ARG_ENABLE([memory-checking],
     [AS_HELP_STRING([--enable-memory-checking], [enable memory checking])],
     [AC_DEFINE([GAP_MEM_CHECK], [1], [define if building with memory checking])],
@@ -222,6 +243,7 @@ AC_ARG_ENABLE([memory-checking],
 AC_MSG_CHECKING([whether to enable memory checking])
 AC_MSG_RESULT([$enable_memory_checking])
 
+dnl
 AC_ARG_ENABLE([valgrind],
     [AS_HELP_STRING([--enable-valgrind], [enable valgrind extensions to GASMAN])],
     [AC_DEFINE([GAP_MEMORY_CANARY], [1], [define if building with valgrind extensions])],