configure: add support for helper script to compute version string

Some projects are using (or planning to use) crosstool-NG, and are storing
it in their VCS, which might not be Mercurial. At the same time, those
projects may want to track development snapshots versions the way we do
with the Hg identity string (hg id).

Provide a way for these project to do so, without having to patch
./configure, and maintain that patch over-and-over again.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

Original patch from Gentoo.

-= BEGIN original header =-

-= END original header =-

diff -durN uClibc-0.9.30.orig/Rules.mak uClibc-0.9.30/Rules.mak

--- uClibc-0.9.30.orig/Rules.mak	2008-11-12 13:24:16.000000000 +0100

+++ uClibc-0.9.30/Rules.mak	2009-02-07 09:57:59.000000000 +0100

@@ -519,6 +519,7 @@

 endif

 CFLAGS += $(call check_gcc,-std=gnu99,)

+CFLAGS += $(call check_gcc,-fgnu89-inline,)

 LDFLAGS_NOSTRIP:=$(CPU_LDFLAGS-y) -shared \

 	-Wl,--warn-common -Wl,--warn-once -Wl,-z,combreloc