collectd: disallow for static-only builds

collectd doesn't really support static-only builds: each plugin
defines a module_register() function, and plugins are supposed to be
dynamically loaded.

Also, passing -lm in LIBS makes more sense than LDFLAGS, as it is
passed at the end of the build command line.

Fixes:

  http://autobuild.buildroot.org/results/047/047dc20aba46885679b6d21cdd2d77fc252374b6/



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