msys2: Recommend correct cmake package (#59001)
msys2 ships 2 different cmake packages, one built natively (with mingw
prefix in the package name) and one built against the posix emulation
environment. The posix emulation one does not work because it will
detect unix-style paths, which it then writes into files that native
tools process. Unlike during command invocation (where the msys2 runtime
library does path translation), when paths are written to files, they
are written verbatim.
The practical result of this is that e.g. the LLVM build will fail with
a mysterious libz link failure (as e.g. reported in #54981).
This is our fault, because our built instructions tell the user to
install the wrong one.
Fix all that by
1. Correcting the build instructions to install the correct cmake
2. Detecting if the wrong cmake is installed and advising the correct
one
3. Fixing an issue where the native CMake did not like our
CMAKE_C_COMPILER setting.
With all this, CMake runs correctly under msys2 with
USE_BINARYBUILDER_LLVM=0.
