Every policy's documentation has a paragraph on what version of CMake
introduced it, how to set the policy, and whether CMake warns if the
policy is not set. The wording of this paragraph has diverged across
policies over time. Factor the paragraph out into a standard advice
document included by every policy.
Forward `CMAKE_<LANG>_FLAGS` and `CMAKE_<LANG>_FLAGS_DEBUG` from the
calling project into the test project. The set of flags may affect the
availability of IPO support. Since this may change the result of the
check for existing projects, add a policy for compatibility.
This was discovered after commit 5fcadc481e (MSVC: Default to -ZI
instead of /Zi for x86 and x64, 2022-05-24) introduced policy CMP0138 to
switch our default for MSVC's debug info flag. The `-ZI` flag is
incompatible with the `-GL` flag used for IPO, so CMP0138 was reverted
pending future work on an alternative solution. Re-use the CMP0138
policy number for this change to CheckIPOSupported instead.
Fixes: #23607
Revert commit 5fcadc481e (MSVC: Default to -ZI instead of /Zi for x86
and x64, 2022-05-24). The `-ZI` flag is incompatible with the `-GL`
flag used for IPO, and so is not an unconditionally better default.
Revert the change pending future design of a first-class setting for
MSVC debug info format that can be automatically reconciled with IPO
settings.
That commit introduced policy CMP0138, but we already have later policy
numbers used too. Leave placeholder text to avoid policy renumbering.
Issue: #23607, #10189
Alex Turbov