php: make sure either CLI or CGI is selected

When neither CLI nor CGI is selected, PHP's configure script fails
with:

checking whether to build CGI binary... configure: error: No SAPIs selected.

Of couse, the help text of the options says that at least one of them
should be selected, but when doing testing with randpackageconfig,
noone is reading these help texts.

Therefore, based on the suggestion of Yann E. Morin, modify the
Config.in organization so that at least one of the option is selected.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>