package/dhcp: build internal bind tool gen for host (not target)

Building gen tool of the internal (bundled) bind needs to be for host, not target
Switch to use internal build in commit 0c8dd6ebd656e06cf99a63eb93343715f4853503
overlooked this.

Building dns library in /home/autobuild/autobuild/instance-13/output-1/build/dhcp-4.4.2-P1/bind/bind-9.11.14/lib/dns
/bin/sh: line 1: ./gen: cannot execute binary file: Exec format error

Thus, we need to set not just CC, but also CFLAGS etc. otherwise the
target CFLAGS etc. will be inherited from top-level configure.

Fixes:
 - http://autobuild.buildroot.org/results/da6fd904d1a6bae73b6ff89dd008de1f459bb7d7/

Signed-off-by: Tim Hammer <Tim.Hammer@orolia.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/dhcp/dhcp.mk

@@ -23,7 +23,12 @@ DHCP_CONF_ENV = \
 	CPPFLAGS='-D_PATH_DHCPD_CONF=\"/etc/dhcp/dhcpd.conf\" \
 		-D_PATH_DHCLIENT_CONF=\"/etc/dhcp/dhclient.conf\"' \
 	CFLAGS='$(TARGET_CFLAGS) -DISC_CHECK_NONE=1'
-DHCP_BIND_EXTRA_CONFIG = BUILD_CC='$(TARGET_CC)'
+
+DHCP_BIND_EXTRA_CONFIG = \
+	BUILD_CC='$(HOSTCC)' \
+	BUILD_CFLAGS='$(HOST_CFLAGS)' \
+	BUILD_CPPFLAGS='$(HOST_CPPFLAGS)' \
+	BUILD_LDFLAGS='$(HOST_LDFLAGS)'
 
 DHCP_CONF_ENV += ac_cv_prog_AWK=$(HOST_DIR)/bin/gawk