configure: add --enable-shared-sqlite option

When --enable-shared-sqlite is used, charybdis will be compiled with a shared
sqlite library if it is available.

By default, the almagated C file for sqlite3 will be compiled and linked statically unless --enable-shared-sqlite is used with ./configure.

Potential use cases for this include allowing OS distributors to use the shared library, which Debian currently does with a patch (see bug #22) and compiling charybdis on low-memory devices that have a shared sqlite libray available but do not have enough memory to compile sqlite's amalgamated C file (e.g. low-power ARM boards and budget VPSes).