Add standard ipo variant to CMakePackage.

Rationale

CMake exposes the CMAKE_INTERPROCEDURAL_OPTIMIZATION variable to enable IPO/LTO on all targets in a project. Similar to the standard build_type variant for CMake, Spack should have a standard ipo variant (defaulting to False) that enables CMAKE_INTERPROCEDURAL_OPTIMIZATION.

Description

# lib/spack/spack/build_systems/cmake.py
variant('ipo', default=False, description='CMake interprocedural optimization')

def _std_args(pkg):
    ...
    try:
        ipo = pkg.spec.variants['ipo'].value
    except KeyError:
        ipo = False

    args = [
        '-G', generator,
        define('CMAKE_INSTALL_PREFIX', pkg.prefix),
        define('CMAKE_BUILD_TYPE', build_type),
        define_from_variant('CMAKE_INTERPROCEDURAL_OPTIMIZATION', ipo),
    ]

Additional information

Spack 0.15.4-710-125191931

General information

• I have run spack --version and reported the version of Spack
• I have searched the issues of this repo and believe this is not a duplicate