Header checking failures on LLVM-less machines

Since the LLVM stuff went in, src/tools/pginclude/cpluspluscheck
fails on my main devel machine, because

In file included from /tmp/cpluspluscheck.jobsM6/test.cpp:3:
./src/include/jit/llvmjit_emit.h:13:25: error: llvm-c/Core.h: No such file or directory

and then there's a bunch of ensuing spewage due to missing typedefs
etc.  llvmjit.h has the same problem plus this additional pointless
aggravation:

In file included from /tmp/cpluspluscheck.jobsM6/test.cpp:3:
./src/include/jit/llvmjit.h:15:2: error: #error "llvmjit.h should only be included by code dealing with llvm"

I propose that we should fix this by making the whole bodies of those
two headers be #ifdef USE_LLVM.

			regards, tom lane