package/php: fix gd extension dependencies and options

package/php: fix gd extension dependencies and options

Due to migration to pkg-config in php 7.4, the detection of
library dependencies has been changed.

source from php.net:
--with-gd becomes --enable-gd
--with-png-dir has been removed. libpng is required.
--with-zlib-dir has been removed. zlib is required.
--with-freetype-dir becomes --with-freetype
--with-jpeg-dir becomes --with-jpeg

Signed-off-by: Louis Aussedat <aussedat.louis@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>