Expand the variables in PHP_EXPAND_BUILD_VARS.

Expand the variables in PHP_EXPAND_BUILD_VARS.

Make HAVE_ICONV/HAVE_LIBICONV available through the more
fine-grained approach using a single include file per
directive.  This will significantly reduce the price of
full dependencies for developers, because basically each
file today includes php.h which includes php_config.h.

If PHP_ATOM_INC is defined, these include files can be used.

For all PHP_DEFINEs, empty files are created upon configure start.