PROTON-1333: CMake error if no C++ compiler avaliable
PROTON-1333: CMake error if no C++ compiler avaliable

Change if() statement to use variable name not quoted ${variable}.

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