package/libkrb5: fix BR2_SHARED_STATIC_LIBS build

Fix the following build failure with BR2_SHARED_STATIC_LIBS raised since
the addition of the package in commit
7ae94b51ed5d51fb9fa3f5a931bb8435bcbfaa42 and
https://github.com/krb5/krb5/commit/bd13166a4098198832d921b8e50b97008b714b7b

configure: error: --enable-static must be specified with --disable-shared

Fixes:
 - http://autobuild.buildroot.org/results/dadbf9f77ffdcc4d4e48c6d7e6beb5da303a8074

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/libkrb5/libkrb5.mk

@@ -37,6 +37,11 @@ LIBKRB5_CONF_OPTS = \
 	--without-tcl \
 	--disable-rpath
 
+# Enabling static and shared at the same time is not supported
+ifeq ($(BR2_SHARED_STATIC_LIBS),y)
+LIBKRB5_CONF_OPTS += --disable-static
+endif
+
 ifeq ($(BR2_PACKAGE_OPENLDAP),y)
 LIBKRB5_CONF_OPTS += --with-ldap
 LIBKRB5_DEPENDENCIES += openldap