configure: Define NO_JUMP_TABLE if all we have is llvm-gcc

Commit dd24ca1cb76d attempts to fix the problem that LLVM-based
compilers (such as llvm-gcc-4.2 and clang) miscompiles beam_emu.c.
The idea was to force the use of gcc-4.2 if the default compiler
was LLVM-based.

Since that fix, Apple released Xcode 4.2 that does not include
any version of gcc, only llvm-gcc-4.2 and clang.

We could require gcc in order to be the system, but it would be
nice if Erlang/OTP could be built out-of-box on MacOS X, albeit
with reduced performance.

Therefore, make sure that we set NO_JUMP_TABLE (use a switch
statement instead of computed gotos in beam_emu.c) if no compiler
that correctly handles computed gotos can be found.

We know that clang based on the upcoming LLVM 3.0 will work, but older
LLVM-based compilers will not, so we can test the version of clang.
llvm-gcc has been discontinued in LLVM 3.0, so if the compiler is
LLVM-based but not clang, we can assume that it does not handles
computed gotos correctly.

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