libfcgi: fix build problem of libfcgi++

Fix the way libfcgi++ is linked against libfcgi to solve the
http://autobuild.buildroot.org/results/f8e88e93f32e4e94ac9aef5a3f3f2826f1970453/build-end.log


build problem.

Since we're fixing Makefile.am, take that opportunity to add -lm where
needed, to avoid the need of using custom LDFLAGS in libfcgi.mk.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>