FS-353: Don't add linker flags from pkg-config as make targets
FS-5681: Allow a system provided version of pcre to be used
FS-353: Allow a system provided version of Speex to be used
FS-353: Allow a system provided version of libedit to be used
FS-353: Allow a system provided version of SQLite to be used

Still uses the bundled hash.c because SQLite's hash functions are
normally private and recent versions lack the "copyKey" feature that
FreeSWITCH relies on. hash.c is built independently from SQLite with
the help of a small additional file, system_sqlite.c.
WIP: Build debian with optional system libs

Note: Just for testing; we're not actually going to build with system
sqlite.

FS-353