Add IPO support for Visual Studio (which is referred to by VS as
"Link Time Code Generation" and "Whole Program Optimization"), for
VS version >= 10. This allows CMake/VS users to enable IPO by setting
property `INTERPROCEDURAL_OPTIMIZATION`.
Fixes: #16748
Avoid passing the output of our `try_compile` through a macro argument.
Macro invocations re-parse their arguments and so should not be given
arbitrary content that may contain backslashes and such.
Instead pass a simple message. This is also more readable in the case
that the macro generates a message itself. After the macro returns,
report the real output directly to the variable named by our caller.
Also record the output in a log file as other checks do on failure.
Fixes: #18244
Simplify the test for whether a `RESULT` argument was given to the
`check_ipo_supported` call. Also do not set an empty variable name
if `OUTPUT` was not given.
We only define `INTERPROCEDURAL_OPTIMIZATION` behavior for C, CXX, and
Fortran languages. Do not try to enable support for other languages.
Furthermore, each language builds with a different compiler, so check
for support by CMake and the compiler for each language independently.
Fixes: #16944
Previously the `INTERPROCEDURAL_OPTIMIZATION` target property was
honored only for the Intel compiler on Linux and otherwise ignored. In
order to add support for more compilers incrementally without changing
behavior in the future, add a new policy whose NEW behavior enforces the
`INTERPROCEDURAL_OPTIMIZATION` property. Add flags for supported
compilers and otherwise produce an error.
