Stop building libs that already exist on the system

If we already have appropriate versions of:

 * fastlz
 * libsqlite3
 * lz4
 * double-conversion

then we don't need to build them.

This change also lessens the required patches for distro
inclusion in both Debian and Fedora

I'm not really sure if the logic is correct in the
double-conversion header installation. Please check it.