add option to install or not the static library

[remicollet]

New version of #665

This fixes an issue for downstream distribution, where static libraries are discouraged

For now (v0.10.0) we have to use BUILD_TESTS + BUILD_STATIC_LIBS to be able to run the test suite
and to exclude librabbitmq.a from the package

BUT installed cmake files contain references to the static library, so are (probably) unusable

This trivial fix allow to build but not install the static library, thus cmake files are clean.

[alanxz]

I will accept this as a short-term fix.

Long term I intend on making the build system generate either shared or static libraries - so this CMake option may go away before the next tagged release.