Force bundled double-conversion code to be built when 3rdparty package can't be found

On Windows will require vcpkg or Conan for 3rdparty packages for sure, so we can get double-conversion from one of those package managers.

On macOS double-conversion can be found in brew, but it is not strictly required because the libc++ library on macOS provides appropriate sprintf_l functions.

On Linux it looks like distros are moving away from packaging double-conversion and the libstdc++ library does not provide the required functions that QtCore needs.

Hence we should still use the bundled sources at least for the Linux case.