tools/m4: add AC_LANG_SOURCE to fix autoconf warnings

tools/m4: add AC_LANG_SOURCE to fix autoconf warnings

I see these warnings with autoconf 2.68, add AC_LANG_SOURCE as suggested
by upstream documentation.

...
 # bash autogen.sh
configure.ac:141: warning: AC_LANG_CONFTEST: no AC_LANG_SOURCE call detected in body
../../lib/autoconf/lang.m4:194: AC_LANG_CONFTEST is expanded from...
../../lib/autoconf/general.m4:2662: _AC_LINK_IFELSE is expanded from...
../../lib/autoconf/general.m4:2679: AC_LINK_IFELSE is expanded from...
../../lib/m4sugar/m4sh.m4:606: AS_IF is expanded from...
../../lib/autoconf/general.m4:2032: AC_CACHE_VAL is expanded from...
../../lib/autoconf/general.m4:2053: AC_CACHE_CHECK is expanded from...
m4/pthread.m4:21: AX_CHECK_PTHREAD is expanded from...
configure.ac:141: the top level
configure.ac:142: warning: AC_LANG_CONFTEST: no AC_LANG_SOURCE call detected in body
../../lib/autoconf/lang.m4:194: AC_LANG_CONFTEST is expanded from...
../../lib/autoconf/general.m4:2662: _AC_LINK_IFELSE is expanded from...
../../lib/autoconf/general.m4:2679: AC_LINK_IFELSE is expanded from...
../../lib/m4sugar/m4sh.m4:606: AS_IF is expanded from...
../../lib/autoconf/general.m4:2032: AC_CACHE_VAL is expanded from...
../../lib/autoconf/general.m4:2053: AC_CACHE_CHECK is expanded from...
m4/ptyfuncs.m4:1: AX_CHECK_PTYFUNCS is expanded from...
configure.ac:142: the top level
configure.ac:141: warning: AC_LANG_CONFTEST: no AC_LANG_SOURCE call detected in body
../../lib/autoconf/lang.m4:194: AC_LANG_CONFTEST is expanded from...
../../lib/autoconf/general.m4:2662: _AC_LINK_IFELSE is expanded from...
../../lib/autoconf/general.m4:2679: AC_LINK_IFELSE is expanded from...
../../lib/m4sugar/m4sh.m4:606: AS_IF is expanded from...
../../lib/autoconf/general.m4:2032: AC_CACHE_VAL is expanded from...
../../lib/autoconf/general.m4:2053: AC_CACHE_CHECK is expanded from...
m4/pthread.m4:21: AX_CHECK_PTHREAD is expanded from...
configure.ac:141: the top level
configure.ac:142: warning: AC_LANG_CONFTEST: no AC_LANG_SOURCE call detected in body
../../lib/autoconf/lang.m4:194: AC_LANG_CONFTEST is expanded from...
../../lib/autoconf/general.m4:2662: _AC_LINK_IFELSE is expanded from...
../../lib/autoconf/general.m4:2679: AC_LINK_IFELSE is expanded from...
../../lib/m4sugar/m4sh.m4:606: AS_IF is expanded from...
../../lib/autoconf/general.m4:2032: AC_CACHE_VAL is expanded from...
../../lib/autoconf/general.m4:2053: AC_CACHE_CHECK is expanded from...
m4/ptyfuncs.m4:1: AX_CHECK_PTYFUNCS is expanded from...
configure.ac:142: the top level

Please rerun autoconf after applying this.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>