package/imagemagick: fix host build for old distributions

Fixes:
http://autobuild.buildroot.net/results/5f0/5f0b85033e800c9eebc46812592966ec6826bb5d/

imagemagick uses clock_gettime, which was provided by librt rather than libc
in glibc < 2.17 - Causing link errors.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/imagemagick/imagemagick.mk

@@ -176,6 +176,9 @@ HOST_IMAGEMAGICK_CONF_OPTS = \
 	--with-png \
 	--with-zlib
 
+# uses clock_gettime, which was provided by librt in glibc < 2.17
+HOST_IMAGEMAGICK_CONF_ENV = LIBS="-lrt"
+
 HOST_IMAGEMAGICK_DEPENDENCIES = \
 	host-libjpeg \
 	host-libpng \