make sprng, gmp problems ignorable

[troy]

bad builds of sprng or gmp can stop the entire build of offline-software because they are used in phys-services. First fix is ticket #63. Second fix (this ticket) is to add SPRNG_FOUND, etc. variables to the Find* cmake macros and omit the source that is dependent on these things.

so USE_TOOLS(sprng) is not an error if SPRNG not found, and

if (SPRNG_FOUND)

set(EXTRA_SOURCES private/phys-services/I3SPRNGRandomService.cxx)

endif(SPRNG_FOUND)

lib(private/phys-services/foo.cxx private/phys-services/bar.cxx ${EXTRA_SOURCES})

like that.

then the more problematic tools could be omitted from the build.