libstatgrab: fix detection of libstatgrab >= 0.90

libstatgrab: fix detection of libstatgrab >= 0.90

Ensure linking is done after compilation of the test program, otherwise
symbols cannot be found. `LDFLAGS` and `CFLAGS` are placed before the
test program on gcc command line while `LIBS` is placed after. Even if
in our case, `LDFLAGS` also contain the lib, being put before the
program while the linking phase is done with the same command leads to
`sg_init()` never found which is misinterpreted as requiring an
argument.