package/quagga: add optional support for libcap

When libcap was compiled before, quagga will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/vtysh | grep NEEDED | sort
 0x0000000000000001 (NEEDED)             Shared library: [libcap.so.2]
[...]

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

package/quagga/quagga.mk

@@ -14,6 +14,13 @@ QUAGGA_CONF_OPTS = --program-transform-name=''
 # 0002-configure-fix-static-linking-with-readline.patch
 QUAGGA_AUTORECONF = YES
 
+ifeq ($(BR2_PACKAGE_LIBCAP),y)
+QUAGGA_CONF_OPTS += --enable-capabilities
+QUAGGA_DEPENDENCIES += libcap
+else
+QUAGGA_CONF_OPTS += --disable-capabilities
+endif
+
 QUAGGA_CONF_OPTS += $(if $(BR2_PACKAGE_QUAGGA_ZEBRA),--enable-zebra,--disable-zebra)
 QUAGGA_CONF_OPTS += $(if $(BR2_PACKAGE_QUAGGA_BABELD),--enable-babeld,--disable-babeld)
 QUAGGA_CONF_OPTS += $(if $(BR2_PACKAGE_QUAGGA_BGPD),--enable-bgpd,--disable-bgpd)