gnutls: downgrade gettext requirements

* use previous gettext version 0.18.3
* should fix build on Ubuntu Trusty 14.04
* keep gettext 0.19 on windows for mingw compilation

Tuleap: #62
Change-Id: I39f3d5b4938f7b2926f730a3b7822c19e05d9d1e

contrib/src/gnutls/downgrade-gettext-requirement.patch

+--- gnutls-3.4.6/configure.ac	2015-10-07 04:21:22.000000000 -0400
++++ gnutls-3.4.6/configure.ac	2015-11-04 16:17:24.705546543 -0500
+@@ -200 +200 @@
+-AM_GNU_GETTEXT_VERSION([0.19])
++AM_GNU_GETTEXT_VERSION([0.18.3])

contrib/src/gnutls/rules.mak

@@ -17,6 +17,8 @@ gnutls: gnutls-$(GNUTLS_VERSION).tar.xz .sum-gnutls
 	$(UNPACK)
 ifdef HAVE_WIN32
 	$(APPLY) $(SRC)/gnutls/gnutls-win32.patch
+else
+	$(APPLY) $(SRC)/gnutls/downgrade-gettext-requirement.patch
 endif
 ifdef HAVE_ANDROID
 	$(APPLY) $(SRC)/gnutls/no-create-time-h.patch