gnupg: don't prefix assembly level functions with underscore

Fixes http://autobuild.buildroot.net/results/45a0856bafa9f2f7e86e2c063528c2b5b04c08d6

gnupg's configure script defaults to prepending an underscore ('_') to
the assembly level functions, which isn't correct for Linux and causes
linker errors for the archs where it has asm optimizations.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/gnupg/gnupg.mk

@@ -10,6 +10,7 @@ GNUPG_SITE = ftp://ftp.gnupg.org/gcrypt/gnupg
 GNUPG_LICENSE = GPLv3+
 GNUPG_LICENSE_FILES = COPYING
 GNUPG_DEPENDENCIES = zlib ncurses
+GNUPG_CONF_ENV = ac_cv_sys_symbol_underscore=no
 GNUPG_CONF_OPT = --disable-rpath --enable-minimal --disable-regex
 
 ifeq ($(BR2_PACKAGE_BZIP2),y)