Currently, building Geoclue without the NMEA source fails since the core
code calls gclue_nmea_is_gga() and gclue_nmea_is_rmc() functions located in
gclue-nmea-source.c, a file which is not included in such build.

Move these functions instead to a new gclue-nmea-utilc.c file, which gets
compiled unconditionally (together with accompanying simple
gclue_nmea_is_nmea() for consistency).

Closes #137