Add LIBDIR variable for the musl library directory

LIBDIR should be used where possible to avoid unnecessary
duplication in build scripts that target the musl toolchain.

No change in package hashes.

sysc/python-2.5.6/python-2.5.6.sh

@@ -44,12 +44,12 @@ src_prepare() {
 src_configure() {
     MACHDEP=linux ac_sys_system=Linux \
     CFLAGS="-U__DATE__ -U__TIME__" \
-    LDFLAGS="-L/usr/lib/musl" \
+    LDFLAGS="-L${LIBDIR}" \
         ./configure \
         --build=i386-unknown-linux-musl \
         --host=i386-unknown-linux-musl \
         --prefix="${PREFIX}" \
-        --libdir="${PREFIX}/lib/musl" \
+        --libdir="${LIBDIR}" \
         --with-system-ffi \
         --enable-ipv6
 }