lib-httpd: extend module location auto-detection

Although it is possible to manually set LIB_HTTPD_PATH and LIB_HTTPD_MODULE_PATH to point at the location of `httpd` and its modules, doing so is cumbersome and easily forgotten. To address this, 0d344738dc (t/lib-http.sh: Restructure finding of default httpd location, 2010-01-02) enhanced lib-httpd.sh to automatically detect the location of `httpd` and its modules in order to facilitate out-of-the- box testing on a wider range of platforms. Follow that lead by further enhancing it to automatically detect the `httpd` modules on Void Linux, as well. Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>