aclocal.m4: allow override of dllwrap and windres when cross-compiling
aclocal.m4: allow override of dllwrap and windres when cross-compiling

Commit 66108864540601837ad77847f4062a670362361f reverted ability
to override 'dllwrap' and 'windres' paths when cross-compiling.

After this change (and a few libraries/Win32 patches) I was able
to build cross-compiler to windows:

    $ ./configure --target=i686-w64-mingw32 \
        DllWrap=i686-w64-mingw32-dllwrap \
        Windres=i686-w64-mingw32-windres

Later both 'dllwrap' and 'windres' should be derived from --target=
as we do now for 'CC', 'AR', 'NM' and others.

Signed-off-by: Sergei Trofimovich <110fdadc76b6bce16977f568bdb4723eaed6fb3f@gentoo.org>

shlevy/ghc,ezyang/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc