clang plugin: allow overriding llvm-config binary
clang plugin: allow overriding llvm-config binary

ppa:h-rayflood/llvm has a llvm-3.2 (and clang) with the llvm-config binary named llvm-config-3.2 instead (that is, version suffixed); this allows building the clang plugin with `make LLVM_CONFIG=llvm-config-3.2` to succeed (since the package doesn't use update-alternatives for some reason).

This does not change the default behaviour.
jbradberry/dxr,erikrose/dxr,pombredanne/dxr,jbradberry/dxr,erikrose/dxr,bozzmob/dxr,jonasfj/dxr,bozzmob/dxr,gartung/dxr,gartung/dxr,KiemVM/Mozilla--dxr,kleintom/dxr,gartung/dxr,srenatus/dxr,gartung/dxr,erikrose/dxr,srenatus/dxr,srenatus/dxr,jay-z007/dxr,nrc/dxr,pombredanne/dxr,nrc/dxr,KiemVM/Mozilla--dxr,kleintom/dxr,jay-z007/dxr,KiemVM/Mozilla--dxr,KiemVM/Mozilla--dxr,jonasfj/dxr,srenatus/dxr,jonasfj/dxr,pelmers/dxr,kleintom/dxr,pelmers/dxr,pelmers/dxr,KiemVM/Mozilla--dxr,jay-z007/dxr,bozzmob/dxr,pelmers/dxr,jay-z007/dxr,jay-z007/dxr,jay-z007/dxr,jbradberry/dxr,jbradberry/dxr,bozzmob/dxr,bozzmob/dxr,jbradberry/dxr,kleintom/dxr,bozzmob/dxr,pombredanne/dxr,KiemVM/Mozilla--dxr,jay-z007/dxr,pelmers/dxr,nrc/dxr,erikrose/dxr,bozzmob/dxr,pombredanne/dxr,pelmers/dxr,srenatus/dxr,erikrose/dxr,jonasfj/dxr,pombredanne/dxr,kleintom/dxr,kleintom/dxr,gartung/dxr,nrc/dxr,jonasfj/dxr,pombredanne/dxr,pelmers/dxr,kleintom/dxr,jbradberry/dxr,nrc/dxr,gartung/dxr,jonasfj/dxr,gartung/dxr,pombredanne/dxr,jbradberry/dxr,srenatus/dxr,nrc/dxr