To handle safely the values used by CMake variables and properties,
introduce the class cmProp as a replacement from the simple pointer
to std::string instance.
#pragma once is a widely supported compiler pragma, even though it is
not part of the C++ standard. Many of the issues keeping #pragma once
from being standardized (distributed filesystems, build farms, hard
links, etc.) do not apply to CMake - it is easy to build CMake on a
single machine. CMake also does not install any header files which can
be consumed by other projects (though cmCPluginAPI.h has been
deliberately omitted from this conversion in case anyone is still using
it.) Finally, #pragma once has been required to build CMake since at
least August 2017 (7f29bbe6 enabled server mode unconditionally, which
had been using #pragma once since September 2016 (b13d3e0d)). The fact
that we now require C++11 filters out old compilers, and it is unlikely
that there is a compiler which supports C++11 but does not support
#pragma once.
Previously we disallowed use of arbitrary properties on INTERFACE
libraries. The goal was to future-proof projects using them by not
allowing properties to be set that may affect their future inclusion in
the generated buildsystem. In order to prepare to actually include
INTERFACE libraries in the generated buildsystem, drop the filter and
allow arbitrary properties to be set.
Issue: #19145
