build-mingw-w64-libraries: Add an explicit --libdir parameter

On some distributions, e.g. openSUSE, the libraries for 64 bit
architectures end up stored in <target>/<triplet>/lib64 instead of
plain "lib".

(This is caused by their use of a site script for configure, which
overrides libdir when the host triplet matches 64 bit architecture
names - even if it's a triple for cross compilation for a totally
different OS. This only affects the libraries and not mingw-w64-crt,
as the crt's makefiles manually set the output directory name.)

This fixes https://github.com/mstorsjo/llvm-mingw/issues/51.

build-mingw-w64-libraries.sh

@@ -22,7 +22,7 @@ for lib in winpthreads winstorecompat; do
     for arch in $ARCHS; do
         mkdir -p build-$arch
         cd build-$arch
-        ../configure --host=$arch-w64-mingw32 --prefix=$PREFIX/$arch-w64-mingw32
+        ../configure --host=$arch-w64-mingw32 --prefix=$PREFIX/$arch-w64-mingw32 --libdir=$PREFIX/$arch-w64-mingw32/lib
         make -j$CORES
         make install
         cd ..