toolchain-funcs.eclass: Use host vars in tc-getBUILD* when not cross

Make tc-getBUILD* functions respect host variables (CC & co.) when
not cross-compiling. This removes the necessity of overriding BUILD_*
along with the regular variables on the systems that are not concerned
about cross-compilation, and does not change the behavior for those
which are.

Closes: https://bugs.gentoo.org/630282

eclass/toolchain-funcs.eclass

@@ -1,4 +1,4 @@
-# Copyright 1999-2017 Gentoo Foundation
+# Copyright 1999-2018 Gentoo Foundation
 # Distributed under the terms of the GNU General Public License v2
 
 # @ECLASS: toolchain-funcs.eclass
@@ -40,7 +40,13 @@ _tc-getPROG() {
 	export ${var}="${prog[*]}"
 	echo "${!var}"
 }
-tc-getBUILD_PROG() { _tc-getPROG CBUILD "BUILD_$1 $1_FOR_BUILD HOST$1" "${@:2}"; }
+tc-getBUILD_PROG() {
+	local vars="BUILD_$1 $1_FOR_BUILD HOST$1"
+	# respect host vars if not cross-compiling
+	# https://bugs.gentoo.org/630282
+	tc-is-cross-compiler || vars+=" $1"
+	_tc-getPROG CBUILD "${vars}" "${@:2}"
+}
 tc-getPROG() { _tc-getPROG CHOST "$@"; }
 
 # @FUNCTION: tc-getAR