php: convert to Makefile.autotools.in and other improvements

Patch by Gustavo Zacarias <gustavo@zacarias.com.ar>

Closes #5, #77, #141 and #143.

* Migrate php package to Makefile.autotools.in

* Make PCRE regex an option (previously forced on)

* Add calendar option from bug #77

* Add external sqlite3 option from bug #141
New option BR2_PACKAGE_PHP_PDO_SQLITE_EXTERNAL

* Add alternate php.ini configuration option from bug #5
Named BR2_PACKAGE_PHP_CONFIG and defaults to shipped config.
In most scenarios the shipped config isn't good enough, it has
a high memory limit for embedded for example.

* Changed some options from depends to select
Namely openssl, libxml2, zlib, gettext, gmp.

* Disabled some 'y' defaults to make things lighter by default
Namely libxml2, sqlite, pdo and pdo_sqlite.

* Made some of the extensions help text more verbose