Update Makefile for Mac

[djberg96]

I think I found a couple issues in the Makefile (originally on a Mac, hence the title, but I think these are generic).

The first is that line 19 is referring to $(CRYSTAL_ROOT)/llvm/ext/find-llvm-config instead of $(CRYSTAL_ROOT)/llvm/ext/find-llvm-config.sh.

The second is that cxxflags aren't appended.

Prior to these changes I was getting:

>make                                                                                                                      [18:41]
c++  -I/opt/homebrew/opt/openjdk/include  -c -o /Users/dberger/Dev/ic/share/crystal-ic/src/llvm/ext/llvm_ext.o /Users/dberger/Dev/ic/share/crystal-ic/src/llvm/ext/llvm_ext.cc
/Users/dberger/Dev/ic/share/crystal-ic/src/llvm/ext/llvm_ext.cc:1:10: fatal error: 'llvm/Config/llvm-config.h' file not found
    1 | #include <llvm/Config/llvm-config.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
make: *** [/Users/dberger/Dev/ic/share/crystal-ic/src/llvm/ext/llvm_ext.o] Error 1

[zw963]

good catch!