Should we automatically infer CHPL_LLVM_GCC_INSTALL_DIR?

[mppf]

PR # #25913 added CHPL_LLVM_GCC_INSTALL_DIR as an alternative to CHPL_LLVM_GCC_PREFIX. We do infer CHPL_LLVM_GCC_PREFIX if it is not set. Inferring CHPL_LLVM_GCC_PREFIX was added originally in #11615 and PR #18551 updated it to the current form.

CHPL_LLVM_GCC_PREFIX has the problem on some systems that it does not distinguish between different versions of gcc/g++ installed to the same path. For example, Ubuntu 24.04 allows multiple gcc installations that are all stored in the prefix /usr (the version is a suffix on commands in /usr/bin and stored in /usr/lib/gcc/x86_64-linux-gnu/<version>).

As a result, CHPL_LLVM_GCC_INSTALL_DIR is more robust. Should the inference we have, which is based on what gcc in the path uses, cause CHPL_LLVM_GCC_INSTALL_DIR to be set rather than CHPL_LLVM_GCC_PREFIX?