mozcoreconf-v4.eclass: honour toolchain even for build tools

HOST_CC and HOST_CXX are used to build tools like nsinstall which are needed
to install later on.  Until now, these values were undefined and for whatever
reason were never autodetected to match the CHOST based tools.  This commit
should resolve that issue.

Bug: http://bugs.gentoo.org/565124

eclass/mozcoreconf-v4.eclass

@@ -15,7 +15,7 @@
 # This is an eclass-generated variable that defines the rpath that the mozilla
 # product will be installed in.  Read-only
 
-if [[ ! ${_MOZCORECONF_V3} ]]; then
+if [[ ! ${_MOZCORECONF} ]]; then
 
 PYTHON_COMPAT=( python2_7 )
 PYTHON_REQ_USE='ncurses,sqlite,ssl,threads'
@@ -86,6 +86,8 @@ moz_pkgsetup() {
 	export LC_CTYPE="C"
 
 	# Ensure we use correct toolchain
+	export HOST_CC="$(tc-getBUILD_CC)"
+	export HOST_CXX="$(tc-getBUILD_CXX)"
 	tc-export CC CXX LD PKG_CONFIG
 
 	# Ensure that we have a sane build enviroment
@@ -263,5 +265,5 @@ mozconfig_final() {
 	echo "ac_add_options --enable-extensions=${exts// /,}" >> .mozconfig
 }
 
-_MOZCORECONF_V3=1
+_MOZCORECONF=1
 fi