ext/dba/config.m4: Autodetect Berkeley DB v5.3.

ext/dba/config.m4: Autodetect Berkeley DB v5.3.

When the user passes --with-db4=DIR to ./configure, the directory DIR
is searched for a number of paths in order of preference. These paths
contain possible locations for Berkeley DB (BDB) to be installed, and
since PHP only supports BDB-4.x and BDB-5.x, it's important that (for
example) version 5.1 be autodetected before falling back to
/usr/include/db.h which can point to BDB-6.x.

The newer BDB-5.3 works with PHP, and if only BDB-5.3 is installed on
the user's system, the current ./configure script is fine. However, if
both BDB-5.3 and BDB-6.0 are installed, version 5.3 is not
autodetected, causing BDB-6.0 to be used. The ./configure step then fails.

This commit adds autodetection for BDB-5.3, and fixes a bug reported
on Gentoo at https://bugs.gentoo.org/show_bug.cgi?id=564824.