Summary:
Issue reported by @ryandesign and @MarcusCalhoun-Lopez.
CMake doesn't support spaces in flags. This caused older versions of gcc to
ignore the unknown flag "-z noexecstack" on MacOS since it was interpreted as a
single flag, not two separate flags. Then, during compilation it was treated as
"-z" "noexecstack", which was correctly forwarded to the linker. But the MacOS
linker does not support `-z noexecstack` so compilation failed.
The fix is to use `-Wl,-z,noexecstack`. This is never misinterpreted by a
compiler. However, not all compilers support this syntax to forward flags to the
linker. To fix this issue, we check if all the relevant `noexecstack` flags have
been successfully set, and if they haven't we disable assembly.
See also PR#4056 and PR#4061. I decided to go a different route because this is
simpler. It might not successfully set these flags on some compilers, but in that
case it also disables assembly, so they aren't required.
Test Plan:
```
mkdir build-cmake
cmake ../build/cmake/CMakeLists.txt
make -j
```
See that the linker flag is successfully detected & that assembly is enabled.
Run the same commands on MacOS which doesn't support `-Wl,-z,noexecstack` and see
that everything compiles and that `LD_FLAG_WL_Z_NOEXECSTACK` and
`ZSTD_HAS_NOEXECSTACK` are both false.
This fixes compilation with clang-cl on Windows. There
is a bug in cmake so that check_linker_flag() doesn't give
the correct result when using link.exe/lld-link.exe.
Details in CMake's gitlab: https://gitlab.kitware.com/cmake/cmake/-/issues/22023Fixes#3522
fix#3500
CMake 3.18 or later was required by #3392. Because it uses
`CheckLinkerFlag`. But requiring CMake 3.18 or later is a bit
aggressive. Because Ubuntu 20.04 LTS still uses CMake 3.16.3:
https://packages.ubuntu.com/search?keywords=cmake
This change disables `-z noexecstack` check with old CMake. This will
not break any existing users. Because users who need `-z noexecstack`
must already use CMake 3.18 or later.
When compiling without c++ enabled, some variables are not present.
This is a check enforced in recent Cmake versions.
CMake Error at CMakeModules/AddZstdCompilationFlags.cmake:54 (list):
list sub-command REMOVE_DUPLICATES requires list to be present.
Call Stack (most recent call first):
CMakeLists.txt:53 (ADD_ZSTD_COMPILATION_FLAGS)
Ancient versions of CMake required else(), endif(), and similar block
termination commands to have arguments matching the command starting the block.
This is no longer the preferred style.
