PROTON-1798: [ruby] Use CMAKE_C_FLAGS to build ruby module
PROTON-1798: [ruby] Use CMAKE_C_FLAGS to build ruby module

Don't overwrite CMAKE_C_FLAGS when building ruby SWIG modules,
just remove the troublesome -fvisibility=* but keep the rest.

ChugR/qpid-proton,gemmellr/qpid-proton,apache/qpid-proton,ChugR/qpid-proton,kgiusti/qpid-proton,apache/qpid-proton,kgiusti/qpid-proton,alanconway/qpid-proton,apache/qpid-proton,gemmellr/qpid-proton,astitcher/qpid-proton,kgiusti/qpid-proton,kgiusti/qpid-proton,kgiusti/qpid-proton,apache/qpid-proton,astitcher/qpid-proton,gemmellr/qpid-proton,astitcher/qpid-proton,astitcher/qpid-proton,ssorj/qpid-proton,ChugR/qpid-proton,ChugR/qpid-proton,astitcher/qpid-proton,ChugR/qpid-proton,alanconway/qpid-proton,apache/qpid-proton,alanconway/qpid-proton,kgiusti/qpid-proton,alanconway/qpid-proton,ssorj/qpid-proton,gemmellr/qpid-proton,ssorj/qpid-proton,gemmellr/qpid-proton,ChugR/qpid-proton,ssorj/qpid-proton,ssorj/qpid-proton,ssorj/qpid-proton,alanconway/qpid-proton,astitcher/qpid-proton,apache/qpid-proton,gemmellr/qpid-proton,alanconway/qpid-proton