Fix build issue in Makefile (#827)
Fix build issue in Makefile (#827)

The fix introduced in #824 added a stray slash in the build commands when
llvm.include was empty, which caused build failures.
mkfifo/ponyc,boemmels/ponyc,doublec/ponyc,doublec/ponyc,ponylang/ponyc,sgebbie/ponyc,mkfifo/ponyc,ponylang/ponyc,cquinn/ponyc,kulibali/ponyc,malthe/ponyc,malthe/ponyc,Theodus/ponyc,Perelandric/ponyc,jemc/ponyc,kulibali/ponyc,boemmels/ponyc,boemmels/ponyc,lukecheeseman/ponyta,doublec/ponyc,sgebbie/ponyc,Perelandric/ponyc,Theodus/ponyc,lukecheeseman/ponyta,cquinn/ponyc,ryanai3/ponyc,Theodus/ponyc,ponylang/ponyc,jemc/ponyc,sgebbie/ponyc,malthe/ponyc,jupvfranco/ponyc,boemmels/ponyc,Praetonus/ponyc,mkfifo/ponyc,jupvfranco/ponyc,dipinhora/ponyc,kulibali/ponyc,Perelandric/ponyc,Praetonus/ponyc,jupvfranco/ponyc,malthe/ponyc,cquinn/ponyc,kulibali/ponyc,mkfifo/ponyc,dipinhora/ponyc,CausalityLtd/ponyc,Perelandric/ponyc,Theodus/ponyc,jemc/ponyc,jupvfranco/ponyc,CausalityLtd/ponyc,Theodus/ponyc,sgebbie/ponyc,dipinhora/ponyc,boemmels/ponyc,sgebbie/ponyc,ryanai3/ponyc,Perelandric/ponyc,mkfifo/ponyc,Praetonus/ponyc,ryanai3/ponyc,cquinn/ponyc,Praetonus/ponyc,lukecheeseman/ponyta,jupvfranco/ponyc,CausalityLtd/ponyc