autotools.mk: Add PKG_LIBTOOL_PATHS variable.

PKG_LIBTOOL_PATHS can be used to specify to tell libtool_fixup where to look
for libtool scripts of a package. This is useful if a package has it's libtool
script in a non-standard location or has multiple libtool scripts.

The value should be a space seperated list of paths which are relative to
$(PKG_BUILD_DIR). It defautls to $(CONFIGURE_PATH).

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@19713 3c298f89-4303-0410-b956-a3cf2f4a3e73