Implicit DSO Linking

When I use the attached patch, the RPM build process fails on the Fedora development version:

LC_ALL=C /usr/bin/intltool-merge -d -u -c ./po/.intltool-merge-cache ./po data/gpx-viewer.desktop.in data/gpx-viewer.desktop
Generating and caching the translation database
Merging translations into data/gpx-viewer.desktop.
gpx-graph.vala:54.14-54.14: error: only automatic properties can have default values
   default = 4;
             ^
Compilation failed: 1 error(s), 0 warning(s)
make[2]: *** [gpx_viewer_vala.stamp] Error 1
make[2]: *** Waiting for unfinished jobs....
src/gpx-viewer.c: In function 'route_playback_tick':
src/gpx-viewer.c:461: warning: passing argument 2 of 'gpx_graph_set_highlight' makes integer from pointer without a cast
./gpx.h:166: note: expected 'time_t' but argument is of type 'time_t *'
src/gpx-viewer.c:466: warning: passing argument 2 of 'gpx_graph_set_highlight' makes integer from pointer without a cast
./gpx.h:166: note: expected 'time_t' but argument is of type 'time_t *'
make[2]: Leaving directory `/home/fab/rpmbuild/BUILD/gpx-viewer-0.1.2'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/home/fab/rpmbuild/BUILD/gpx-viewer-0.1.2'
make: *** [all] Error 2
error: Bad exit status from /var/tmp/rpm-tmp.Py2Htc (%build)

RPM build errors:
    Bad exit status from /var/tmp/rpm-tmp.Py2Htc (%build)

vala broke api..
fixed in bzr..

How is this relevant for this bug?

Linking against libm explicitly should fix the building issue on Fedora.

Again how is this relevant for this bug?

The error you pasted is because vala broke/changed it's language syntax. There is a workaround (update) for this in bzr.
It is a completely unrelated error, (not caused by something wrong in gpx-viewer) from the bug report.
To fix it you can just remove the default = 4 line from the source code.