Allow the location of "llvm-config" to be overridden on the "make" command line.

This commit adds an LLVM_CONFIG variable to the Makefile, and uses it rather
than directly referencing llvm-config. This allows the value to be changed on
the make command line. For example, on my system where I have installed the llvm
snapshot package for debian, I am running "make LLVM_CONFIG=llvm-config-3.4".
Don't call llvm-config twice when setting LDLIBS.

llvm-config was being called twice when setting LDLIBS: once to get --ldflags,
and another time for "--libs all". It turns out that nothing in "--libs all" is
actually required, so drop that invocation.

Also dropped libstdc++ while I was here, that is not required either.
Specify C99 when building.

I'm seeing complaints about
"warning: ISO C90 does not support ‘long long’ [-Wlong-long]"
when building with GCC 4.7.2, libclang 3.4ish built from r182715.

This commit silences those warnings by telling GCC to build in C99 mode.
Acked-by: Sebastian Sturm <sturm@itp.uni-leipzig.de>