Check for OpenMP compiler support

[PhilipFackler]

Proposed changes

Added find_package call and meaningful error message.
Closes #4372

What type(s) of changes does this code introduce?

• Build related changes

Does this introduce a breaking change?

• No

What systems has this change been tested on?

Ubuntu 22.04

Checklist

• Yes. This PR is up to date with current the current state of 'develop'
• No. Code added or changed in the PR has been clang-formatted
• No. This PR adds tests to cover any new code, or to catch a bug that is being fixed
• No. Documentation has been added (if appropriate)

[williamfgc]

Since OpenMP is a critical part of QMCPACK simulations I wonder if it's best to suggest finding compiler support for it. @prckent and @ye-luo thoughts?

[prckent]

Will push an edit

[prckent]

Have you been able to test this? e.g. Does the fruit company's compiler result in this error?

[prckent]

I haven't tested this code, but I think that this OpenMP availability test will need to be higher up. e.g. if the libstdc++ check has -fopenmp on the compile line, it will already have failed before we reach this point. The OpenMP test should be one of the first run.

[ye-luo]

Please add a comment before this line saying

# OpenMP package currently is only used to to verify compiler support. No feature of this package is been used and we customize OpenMP options directly in per vendor configurations.

[prckent]

This test needs to go before the Testlibstdc++ check, otherwise that can fail for the wrong reason. After the move, the current AppleClang 14.0.0.14000029 fails on the cxx17 check. According to https://en.cppreference.com/w/cpp/compiler_support/17 this is the correct result since memory resources are not yet implemented. Hopefully this will change shortly.

with QMC_OMP=ON or not specified:

-- QMCPACK adds -fstandalone-debug for Debug builds
-- Could NOT find OpenMP_C (missing: OpenMP_C_FLAGS OpenMP_C_LIB_NAMES) 
-- Could NOT find OpenMP_CXX (missing: OpenMP_CXX_FLAGS OpenMP_CXX_LIB_NAMES) 
-- Could NOT find OpenMP (missing: OpenMP_C_FOUND OpenMP_CXX_FOUND) 
CMake Error at CMakeLists.txt:301 (message):
  No compiler support for OpenMP found.  Switching to a compiler with OpenMP
  support is recommended.  Alternatively, you will need to run CMake
  configure with -DQMC_OMP=OFF


-- Configuring incomplete, errors occurred!

with QMC_OMP=OFF:

-- C++17 standard library support not found or incomplete
compiler is AppleClang 14.0.0.14000029
Compiler detected is <Clang> namely clang++ or a vendor variant (icpx, amdclang++, armclang++).
  If not using libcxx, ensure a GCC toolchain version equal or greater than 9.0 gets picked up. Check with '<Clang> -v'. Or use the --gcc-toolchain compiler option (added to both CMAKE_C_FLAGS and CMAKE_CXX_FLAGS) to point to a newer GCC installation.
  Output of test compile is in cpp17_compile_fail.txt
CMake Error at CMake/TestCxx17Library.cmake:55 (message):
  stopping
Call Stack (most recent call first):
  CMakeLists.txt:310 (include)


-- Configuring incomplete, errors occurred!

[ye-luo]

@prckent I moved the openmp check up. Could you verify if it works for you? If yes. Please proceed to merge this PR.

[prckent]

Can confirm your fix reproduces my local fix.

[prckent]

Test this please