gpsd: proper usage of prefix and DESTDIR

prefix should always be /usr, and destdir must be passed as DESTDIR,
and in the environment, not as a scons argument. Finally, we pass the
sysroot= argument to scons so that it doesn't add -L/usr/lib
parameters when compiling.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>