The `clock_gettime()` function in glibc < 2.17 required linking against
librt. Until #1437 nobody had tried using the CMake build system with
a really old version of glibc (in this case 2.12).
The python build system always linked against librt but the CMake build
system never tried to link against it leading to link failures.
This patch teaches the CMake build system to detect if librt is required
when linking against `clock_gettime()` and adds `rt` as a link
dependency if necessary.
This change means if the user explicitly passes `-DUSE_OPENMP=ON` to
CMake during the first configure and the compiler does not support
OpenMP the configure will fail but if the user doesn't specify it the
build system will automatically enable/disable OpenMP support depending
on whether it is supported by the compiler.
This is an improvement on the previous behaviour because previously we
would just emit a warning if `-DUSE_OPENMP=ON` was passed and the
compiler didn't support OpenMP.
before 4cc2b292c0.
It's useful to be able to control the defaults and CMake's internal
logic for GCC/Clang is simple enough that doing this makes sense.
It would be nice to do the same for MSVC but CMake's internal
logic is more complicated so for now it's better that we just use
CMake's default.
compiler defines and flags when using MSVC.
There are lots of defines and flags that I'm unsure about so in
some cases I've changed the behaviour slightly (if I'm confident
the behaviour in the old build system is wrong) or not added the
flag/define at all but just left comments noting what the old build
system did and why I disagree with the old build system's choices.
This analogous to the `--optimize` flag in the Python/Makefile
build system except that we now support doing LTO with Clang/GCC
as well. However it is probably best to avoid doing LTO with
Clang or GCC for now because I see a bunch of warnings about
ODR violations when building with LTO.
LTO can be enabled with the new `LINK_TIME_OPTIMIZATION` option
which is off by default.
using MSVC. This is set by the old build system but we weren't setting
it. This actually MSVC's default but in an effort to try to behave
more like the old build system we will set it anyway.
CMAKE_CXX_FLAGS_<CONFIG>, CMAKE_<TYPE>_LINKER_FLAGS, and
CMAKE_<TYPE>_LINKER_FLAGS_<CONFIG>.
This is useful for debugging where some flags come from. Now that
we don't explicitly set the defaults it useful to see which default
values we are getting from CMake.
