uacme: fix static builds with BR2_PACKAGE_MBEDTLS_COMPRESSION

Fixes:
http://autobuild.buildroot.net/results/2083eca8030e3f7ed8df388a7c29bc5d672b9512/

Uacme hardcodes the mbedtls linker flags / mbedtls does not provide a .pc
file, so we need to manually also link with zlib if mbedtls is built with
compression support.

Signed-off-by: Nicola Di Lieto <nicola.dilieto@gmail.com>
[Peter: Extend commit message]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/uacme/uacme.mk

@@ -13,12 +13,15 @@ UACME_LICENSE = GPL-3.0+
 UACME_LICENSE_FILES = COPYING
 UACME_DEPENDENCIES = libcurl
 
-UACME_CONF_ENV = ac_cv_prog_cc_c99='-std=gnu99'
+UACME_CONF_ENV = ac_cv_prog_cc_c99='-std=gnu99' LIBS="$(UACME_LIBS)"
 
 ifeq ($(BR2_PACKAGE_GNUTLS),y)
 UACME_CONF_OPTS += --with-gnutls
 UACME_DEPENDENCIES += gnutls
 else ifeq ($(BR2_PACKAGE_MBEDTLS),y)
+ifeq ($(BR2_PACKAGE_MBEDTLS_COMPRESSION),y)
+    UACME_LIBS += -lz
+endif
 UACME_CONF_OPTS += --with-mbedtls
 UACME_DEPENDENCIES += mbedtls
 else ifeq ($(BR2_PACKAGE_OPENSSL),y)