Add explicit SQLITE_ENABLE_FTS3. This fixes the firefox and thunderburd
ports build.

Due to recent changes in sqlite3 even if this flag is actually
implicitly added to the build when the FTS4 option is enabled, it
is not reported as present via the
sqlite3_compileoption_used("SQLITE_ENABLE_FTS3") API call, which
is used by the gecko ports configure script to check for a suitable
sqlite3 library.

Related upstream change: https://sqlite.org/src/info/9a443397a6b110f6

PR: 221361, 219963
Submitted by: Vladimir Krstulja <vlad-fbsd@acheronmedia.com>,

		jbeich@

Approved by: portmgr@ (blanket)
MFH: 2017Q3