m4: Fix acinclude.m4 for underscore detection in the symbol.
* acinclude.m4 (GNUPG_SYS_SYMBOL_UNDERSCORE): Fix for a case with underscore.
Note that our upstream for the impolementation is LT_SYS_SYMBOL_USCORE
in libtool/m4/ltdl.m4.
- GnuPG-bug-id: T7170
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>