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.