Remove unnecessary check of policy CMP0049. The policy can never
trigger on our internally-generated name because it has no variable
references.
The rename in commit 0ed5ce4cd8 (cmTarget: Rename AddSource method for
backward compatibility., 2014-03-17, v3.1.0-rc1~688^2~17) made it look
like this code path depended on CMP0049. Then commit 0e1faa28cb
(cmMakefile: Separate custom command setup from actual creation,
2019-09-14, v3.16.0-rc1~85^2) and commit ea1bed34b2 (cmMakefile: Extract
utilities used for creation of custom commands, 2019-09-21,
v3.16.0-rc1~52^2~1) built additional infrastructure to thread that
dependence through the call stack. Remove it all.
#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.
