error if using Enzyme with gcc/g++

[ZuseZ4]

It would be nicer to check if we actually have a LLVM based compiler, but I am worried of some vendor specific compilers having awkward names. So only check for GNU, since that's 100% unsupported. This should still cover almost all cases and prevents issues like this:
closes #1871

Tested locally:

➜  build git:(recognize-unsupported-gnu) CC=/usr/bin/gcc cmake .. -GNinja  -DEnzyme_DIR=/u/drehwald/prog/Enzyme/enzyme/build/ -DCMAKE_LINKER=lld
project languages: C
C compiler: GNU
CMake Error at /u/drehwald/prog/Enzyme/enzyme/build/EnzymeConfigVersion.cmake:8 (message):
  Enzyme requires an LLVM based C compiler.
Call Stack (most recent call first):
  CMakeLists.txt:5 (find_package)


-- Configuring incomplete, errors occurred!
➜  build git:(recognize-unsupported-gnu) ..
➜  test_find_package git:(recognize-unsupported-gnu) rm -rf build && mkdir build && cd build                                                                
➜  build git:(recognize-unsupported-gnu) CC=clang cmake .. -GNinja  -DEnzyme_DIR=/u/drehwald/prog/Enzyme/enzyme/build/ -DCMAKE_LINKER=lld
-- The C compiler identification is Clang 16.0.5
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /u/drehwald/prog/llvm16/build/bin/clang - skipped
-- Detecting C compile features
-- Detecting C compile features - done
found dir /u/drehwald/prog/Enzyme/enzyme/build/
found 1
imported targets LLVMEnzyme-16;ClangEnzyme-16;ClangEnzymeFlags;LLDEnzyme-16;LLDEnzymeFlags;LLDEnzymeAssumeUnknownNoFree;LLDEnzymeLooseTypeFlags;LLDEnzymePrintTypeFlags;LLDEnzymePrintFlags;LLDEnzymeNoStrictAliasingFlags
-- Configuring done (0.5s)
-- Generating done (0.0s)
-- Build files have been written to: /u/drehwald/prog/Enzyme/enzyme/test/test_find_package/build

[wsmoses]

This should be made into a warning as there are legitimate use cases of non clang compilers, for example if part of the code is not differentiated or has custom derivatives defined, or using an existing Bitcode library

[ZuseZ4]

This should be made into a warning as there are legitimate use cases of non clang compilers.

Sure, done.

[wsmoses]

Can you also print the cmake_c_compiler and the thing you're checking in the warning as well?

[ZuseZ4]

Can you also print the cmake_c_compiler and the thing you're checking in the warning as well?

project languages: C
CMake Warning at /u/drehwald/prog/Enzyme/enzyme/build/EnzymeConfigVersion.cmake:8 (message):
  C compiler ID equals GNU

  C compiler: /usr/bin/gcc

  Using Enzyme without an LLVM based C compiler.
Call Stack (most recent call first):
  CMakeLists.txt:5 (find_package)

[wsmoses]

Excellent, I think that extra info will help folks.