cmake: use configure_file() for creating the .pc file

cmake: use configure_file() for creating the .pc file

Escaping in add_custom_target() seems to depend on the shell used in the cmake
generator and using Ninja on Windows, which uses cmd.exe, results in stray backslashes
in the .pc file.

Instead of going through escaping hell just use configure_file() with the existing
libzstd.pc.in file already used by the simple Makefile based build system.

This fixes the .pc file syntax when building zstd with CMake+Ninja+gcc on Windows.