Some autotools modifications fixing some warnings
Decryption from file to non-pageable memory

Another big step to complete our own SQLite VFS: now the encrypted file
decrypts to a non-pageable memory zone and never touch the disk again
until encryptio again when the SQLiteVFS closes the file.

This require the GCR package, but I have no clue what exactly version is
required, because the gcr_secure_memory development documentation not
mention when this API was introduced.

See Bug #659500 - Prevent the unencrypted database from hitting the disk
https://bugzilla.gnome.org/show_bug.cgi?id=659500