configure.ac: fix gnutls flags without "--with-gnutls=PFX"
configure.ac: fix gnutls flags without "--with-gnutls=PFX"

ghaderer/libmicrohttpd,maru/libmicrohttpd-http2,ghaderer/libmicrohttpd,maru/libmicrohttpd-http2,maru/libmicrohttpd-http2,ghaderer/libmicrohttpd