ci: Fix handling of $PKG_CONFIG_LIBDIR

There are two environment variables that are baked into our
cross-compilation container images at build time, $CONFIGURE_OPTS
and $PKG_CONFIG_LIBDIR: the former contain the options necessary
to convince configure to perform a cross build rather than a
native one, and the latter is necessary so that pkg-config will
locate the .pc files for MinGW libraries. Container images that
are not intended for cross-compilation will not have either one
defined.

The problem is that, while an empty $CONFIGURE_OPTS is completely
harmless, setting $PKG_CONFIG_LIBDIR to an emtpy value will
result in pkg-config not looking in its default search path, thus
not finding any library, and subsequently breaking native builds.

To work around this issue, only pass $PKG_CONFIG_LIBDIR to sudo
when the value is set in the calling environment.

Fixes: 71517ae4db
Signed-off-by: Andrea Bolognani <abologna@redhat.com>

ci/Makefile

@@ -216,12 +216,15 @@ ci-run-command@%: ci-prepare-tree
 	$(CI_ENGINE) run $(CI_ENGINE_ARGS) $(CI_IMAGE_PREFIX)$*$(CI_IMAGE_TAG) \
 		/bin/bash -c ' \
 		$(CI_USER_HOME)/prepare || exit 1; \
+		if test "$$PKG_CONFIG_LIBDIR"; then \
+			pkgconfig_env="PKG_CONFIG_LIBDIR=$$PKG_CONFIG_LIBDIR"; \
+		fi; \
 		sudo \
 		  --login \
 		  --user="#$(CI_UID)" \
 		  --group="#$(CI_GID)" \
 		  CONFIGURE_OPTS="$$CONFIGURE_OPTS" \
-		  PKG_CONFIG_LIBDIR="$$PKG_CONFIG_LIBDIR" \
+		  $$pkgconfig_env \
 		  CI_CONT_SRCDIR="$(CI_CONT_SRCDIR)" \
 		  CI_CONT_BUILDDIR="$(CI_CONT_BUILDDIR)" \
 		  CI_SMP="$(CI_SMP)" \