build: Improve warnings generated from Vala generated C files

Don't include all the warnings about -Wunused*
tracker-extract: Don't clean rules_DATA now we no longer produce it from .in files
libtracker-sparql: Avoid compiler warning and cast const to non-const

Used for traversing a char* converted to from a Vala String. We don't change
the char, just the *.
libtracker-sparql: Use existing escape_string() in Builder

the Builder's _object_string() had it's own duplicate version which was less
comprehensive too.
libtracker-data: Allow calling tracker_db_manager_locale_changed() before init

This fixes an issue introduced in commit fa6317fbabb1a7, where the
direct backend would fail to find the 'db-locale.txt' marker due to the
global 'data_dir' path variable being NULL at the time it called
tracker_db_manager_locale_changed(). It would then report that the
locale had changed from 'unknown' to the system locale, and abort.

https://bugzilla.gnome.org/show_bug.cgi?id=676209
libtracker-data: Remove duplicated version of tracker_db_manager_init_locations()

The return value of g_get_user_data_dir() and friends is constant
through the lifecycle of the process, so there's no need to initialise
paths more than once or in different ways.
libtracker-sparql: Avoid strcmp() build warnings in cursor_get_boolean()
libtracker-miner, libtracker-sparql: Fixed missing setlocale() in unit tests

Due to the new warning we have in place where locale != DB locale, this needs
to be set before the tests will work.