This diff makes proper use of the cmake POSITION_INDEPENDENT_CODE
property to enforce building PIC/PIE.
Because this property does not enforce PIE before version 3.14, the
previous method is kept as a fallback, so the behavior is unchanged.

mkdir buildcmake && cd buildcmake
cmake -GNinja ..
ninja check-security

Check there is no error with both cmake < 3.14 and >= 3.14.

Check it works when cross building for windows (no PIC nor PIE
required), requires D4155 to succeed:

rm -rf *
cmake -GNinja .. -DBUILD_BITCOIN_SEEDER=OFF \
  -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Win64.cmake
ninja check-security