build: fix subst toolexecdir, toolexeclibdir On CentOS 7.4, configure script generated by autogen.sh will output the following values when compiling GCC source code toolexecdir='NONE/$(target_alias)' toolexeclibdir='NONE/$(target_alias)/lib' and cause build error ... -o libffi.la -rpath NONE/riscv64-bs-linux-gnu/lib ... checking for shl_load in -ldld... libtool: link: only absolute run-paths are allowed