php-ext-source-r3.eclass: Introduce PHP_EXT_NEEDED_USE

php-ext-source-r3.eclass: Introduce PHP_EXT_NEEDED_USE

This simplifies the dependencies in an ebuild

@DESCRIPTION:
A list of USE flags to append to each PHP target selected
as a valid USE-dependency string.  The value should be valid
for all targets so USE defaults may be necessary.
Example:
PHP_EXT_NEEDED_USE="mysql?,pdo,pcre(+)"

The PHP dependencies will result in:
php_targets_php7-0? ( dev-lang/php:7.0[mysql?,pdo,pcre(+)] )

Bug: https://bugs.gentoo.org/586446