package/libgpgme: always use gpgrt-config from buildroot staging

gpgme updated its gpg-error.m4 macro files to detect gpgrt-config at
several occasions. Those changes are included since buildroot commit
13f3b8e124 "package/libgpgme: bump to version 1.20.0".

gpgrt-config path needs to be forced in _CONF_ENV to make sure it
will work in all host environments. See the log entry of commit
d7f2d8403e "package/gnupg2: fix build failure when host provides an
old gpgrt-config" [2] for a complete explanation.

[1] https://git.gnupg.org/cgi-bin/gitweb.cgi?p=gpgme.git;a=history;f=m4/gpg-error.m4;h=gpgme-1.20.0
[2] https://git.buildroot.org/buildroot/commit/?id=d7f2d8403ec82f3b1772ec17bf1df8c42987a1f8

Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/libgpgme/libgpgme.mk

@@ -33,6 +33,10 @@ LIBGPGME_CONF_OPTS = \
 	--disable-cpp-test \
 	--enable-languages=$(subst $(space),$(comma),$(LIBGPGME_LANGUAGE_BINDINGS))
 
+# Force the path to "gpgrt-config" (from the libgpg-error package) to
+# avoid using the one on host, if present.
+LIBGPGME_CONF_ENV += GPGRT_CONFIG=$(STAGING_DIR)/usr/bin/gpgrt-config
+
 # Handle argp-standalone or it errors out during build
 ifeq ($(BR2_PACKAGE_ARGP_STANDALONE),y)
 # musl libc does not define error_t in errno.h, but argp.h does.