package/apr-util: fix ldap detection

configure checks if ldap_set_rebind_proc takes 2 or 3 arguments
http://svn.apache.org/viewvc/apr/apr-util/tags/1.6.1/build/apu-conf.m4?revision=1812528&view=markup#l370

It uses the macro APR_TRY_COMPILE_NO_WARNING which adds -Werror to
treat all warnings as errors when gcc is used:
http://svn.apache.org/viewvc/apr/apr/tags/1.6.3/build/apr_common.m4?revision=1812527&view=markup#l504

In some buildroot configs a compiler warning occurs during this check:
http://autobuild.buildroot.net/results/241/241ed78b93ce86c859e175530fa485711ff61615//apr-util-1.6.1/config.log

/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/arm-buildroot-linux-uclibcgnueabihf/sysroot/usr/include/features.h:218:5:
 error: #warning requested reentrant code, but thread support was disabled [-Werror=cpp]
cc1: all warnings being treated as errors

Due to this warning 'ac_cv_ldap_set_rebind_proc_style' is set to two
instead of three leading to a build error later on. This patch forces
ac_cv_ldap_set_rebind_proc_style=three to be inline with openldap which
fixes
http://autobuild.buildroot.net/results/241/241ed78b93ce86c859e175530fa485711ff61615/

This solution was inspired by a discussion on the fink mailinglist:
https://sourceforge.net/p/fink/mailman/message/31720482/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/apr-util/apr-util.mk

@@ -48,6 +48,7 @@ APR_UTIL_CONF_OPTS += --without-sqlite3
 endif
 
 ifeq ($(BR2_PACKAGE_OPENLDAP),y)
+APR_UTIL_CONF_ENV += ac_cv_ldap_set_rebind_proc_style=three
 APR_UTIL_CONF_OPTS += \
 	--with-ldap \
 	--with-ldap-include="$(STAGING_DIR)/usr/include/" \