From the Arch linux build file, I can't believe I missed it as I used it as a guide;
sed -i \
-e "s,^TK_BUILD_LIB_SPEC='-L.*/unix,TK_BUILD_LIB_SPEC='-L/usr/lib," \
-e "s,^TK_SRC_DIR='.*',TK_SRC_DIR='/usr/include'," \
-e "s,^TK_BUILD_STUB_LIB_SPEC='-L.*/unix,TK_BUILD_STUB_LIB_SPEC='-L/usr/lib," \
-e "s,^TK_BUILD_STUB_LIB_PATH='.*/unix,TK_BUILD_STUB_LIB_PATH='/usr/lib," \
${pkgdir}/usr/lib/tkConfig.sh