let clang build llvm & change the order of llvm include path

[crabtw]

• When specifying enable-llvm, it still uses gcc to build llvm.
  Adding a condition, let clang do that work.
• If system has installed llvm and specifies include paths in the CFLAGS, it may fail to link rustllvm.
  In freebsd's case, 3rd-party libs are installed in /usr/local, so we have to add extra paths to find them.
  When compiling rustllvm, it looks up /usr/local/include/llvm/Config/Targets.def not bundled llvm.
  That causes RustWrapper.o contains nonexistent symbols, so I replace -I with -iquote to ensure that include paths of LLVM_CXXFLAGS comes before CFLAGS.

[brson]

Thanks for doing all this porting work! I really appreciate it.

The first change is fine.

After staring at this for a while I think I finally understand the problem the second change is solving (replacing -I with -iquote). Instead of doing it inline with the build command can we perform the substitution when we define LLVM_CXXFLAGS_*? Also please add your comment on why we are doing this to the source code so that the next person to encounter this can understand what is going on.

Additionally, I would prefer that the commit that just fixes indentation be combined with the previous one.

[crabtw]

I reverted previous commits and made changes in the definition of LLVM_CXXFLAGS_*
Thanks you!

[brson]

Thanks!