haskell-cabal.eclass: respect AR option

That allows us to build not-so-broken static libraries
for LTO users. Tuhs usually have environment like:
    CFLAGS=-flto
    AR=${CHOST}-gcc-ar

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

eclass/haskell-cabal.eclass

@@ -31,7 +31,7 @@
 #                  not pull upper versions
 #   test-suite --  add support for cabal test-suites (introduced in Cabal-1.8)
 
-inherit eutils ghc-package multilib
+inherit eutils ghc-package multilib toolchain-funcs
 
 # @ECLASS-VARIABLE: CABAL_EXTRA_CONFIGURE_FLAGS
 # @DESCRIPTION:
@@ -330,6 +330,9 @@ cabal-configure() {
 		cabalconf+=(--ghc-option="$option")
 	done
 
+	# toolchain
+	cabalconf+=(--with-ar="$(tc-getAR)")
+
 	# Building GHCi libs on ppc64 causes "TOC overflow".
 	if use ppc64; then
 		cabalconf+=(--disable-library-for-ghci)