cmake: check llvm-config in separate function, fix cmake error

[BratishkaErik]

Pull request #12136 has greatly improved LLVM search logic for CMake (kudos to @topolarity !). HOWEVER, it caused following regression: https://bugs.gentoo.org/886569. So, when targets is missing for LLVM, it prints:

CMake Error at cmake/Findllvm.cmake:90 (continue):
  A CONTINUE command was found outside of a proper FOREACH or WHILE loop
  scope.
Call Stack (most recent call first):
  cmake/Findllvm.cmake:93 (NEED_TARGET)
  CMakeLists.txt:138 (find_package)

I thought that this function should be macro instead so that this body replaces all occurencies of NEED_TARGET. HOWEVER 😄 , continue() does not work in macro: https://gitlab.kitware.com/cmake/cmake/-/issues/22052.

This PR moves "checking if llvm-config supports all what we need" in separate function (that saves potential error in variable, and if it is not empty, this llvm-config is marked as unsuitable), and slightly touches output, so now errors look like this:

Trying /usr/lib/llvm/15/bin/llvm-config...
This llvm-config is not suitable for us:
This LLVM is missing target AArch64. Zig requires LLVM to be built with all default targets enabled.
Trying another llvm-config...
CMake Error at cmake/Findllvm.cmake:97 (message):
  Suitable llvm-config is not found.
Call Stack (most recent call first):
  CMakeLists.txt:138 (find_package)

and this is normal output:

Trying /usr/lib/llvm/15/bin/llvm-config...
This llvm-config is suitable for us, continuing...

[BratishkaErik]

Oops, I forgot that GREATER_EQUAL was added in CMake 3.7. Perhaps we can bump minimal version? It was released 6 years ago.

[BratishkaErik]

If possible can you add this to 0.10.1 please? Thanks

[andrewrk]

Reverted in b1207b3.