configure: silence autoreconf when not in a git tree

When autoreconfiguring ourside of a git tree, the output is verbose
with git errors.

Silence that by consigning stderr to oblivion.

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

diff -durN gnupg2-2.0.29.orig/configure.ac gnupg2-2.0.29/configure.ac
--- gnupg2-2.0.29.orig/configure.ac	2015-09-08 14:39:24.000000000 +0200
+++ gnupg2-2.0.29/configure.ac	2015-12-16 18:34:47.099493863 +0100
@@ -33,11 +33,11 @@
 # flag indicating a development version (mym4_isgit).  Note that the
 # m4 processing is done by autoconf and not during the configure run.
 m4_define([mym4_revision],
-          m4_esyscmd([git rev-parse --short HEAD | tr -d '\n\r']))
+          m4_esyscmd([git rev-parse --short HEAD 2>/dev/null | tr -d '\n\r']))
 m4_define([mym4_revision_dec],
           m4_esyscmd_s([echo $((0x$(echo ]mym4_revision[|head -c 4)))]))
 m4_define([mym4_betastring],
-          m4_esyscmd_s([git describe --match 'gnupg-2.[0-9].*[0-9]' --long|\
+          m4_esyscmd_s([git describe --match 'gnupg-2.[0-9].*[0-9]' --long 2>/dev/null |\
                         awk -F- '$3!=0{print"-beta"$3}']))
 m4_define([mym4_isgit],m4_if(mym4_betastring,[],[no],[yes]))
 m4_define([mym4_full_version],[mym4_version[]mym4_betastring])