Fix compilation under Linux with g++.
Fix compilation under Linux with g++.

This is a dirty hack, I just removed an option that should have
been added only under clang.

graetzer/arangodb,hkernbach/arangodb,wiltonlazary/arangodb,hkernbach/arangodb,baslr/ArangoDB,fceller/arangodb,Simran-B/arangodb,hkernbach/arangodb,arangodb/arangodb,fceller/arangodb,hkernbach/arangodb,hkernbach/arangodb,m0ppers/arangodb,graetzer/arangodb,m0ppers/arangodb,Simran-B/arangodb,graetzer/arangodb,m0ppers/arangodb,graetzer/arangodb,baslr/ArangoDB,baslr/ArangoDB,m0ppers/arangodb,joerg84/arangodb,graetzer/arangodb,m0ppers/arangodb,baslr/ArangoDB,baslr/ArangoDB,fceller/arangodb,Simran-B/arangodb,joerg84/arangodb,Simran-B/arangodb,joerg84/arangodb,hkernbach/arangodb,wiltonlazary/arangodb,joerg84/arangodb,joerg84/arangodb,baslr/ArangoDB,fceller/arangodb,fceller/arangodb,arangodb/arangodb,fceller/arangodb,graetzer/arangodb,fceller/arangodb,fceller/arangodb,wiltonlazary/arangodb,hkernbach/arangodb,m0ppers/arangodb,m0ppers/arangodb,baslr/ArangoDB,arangodb/arangodb,joerg84/arangodb,baslr/ArangoDB,m0ppers/arangodb,baslr/ArangoDB,arangodb/arangodb,wiltonlazary/arangodb,fceller/arangodb,fceller/arangodb,m0ppers/arangodb,graetzer/arangodb,m0ppers/arangodb,Simran-B/arangodb,arangodb/arangodb,wiltonlazary/arangodb,hkernbach/arangodb,arangodb/arangodb,Simran-B/arangodb,hkernbach/arangodb,graetzer/arangodb,m0ppers/arangodb,joerg84/arangodb,graetzer/arangodb,m0ppers/arangodb,hkernbach/arangodb,arangodb/arangodb,graetzer/arangodb,baslr/ArangoDB,arangodb/arangodb,hkernbach/arangodb,joerg84/arangodb,wiltonlazary/arangodb,baslr/ArangoDB,baslr/ArangoDB,joerg84/arangodb,wiltonlazary/arangodb,hkernbach/arangodb,wiltonlazary/arangodb,graetzer/arangodb,joerg84/arangodb,graetzer/arangodb,joerg84/arangodb,baslr/ArangoDB,m0ppers/arangodb,joerg84/arangodb,hkernbach/arangodb,Simran-B/arangodb,Simran-B/arangodb,joerg84/arangodb,hkernbach/arangodb,graetzer/arangodb,Simran-B/arangodb,graetzer/arangodb,joerg84/arangodb,Simran-B/arangodb,baslr/ArangoDB