AC_CHECK_PROG documentation

[Stepan Kasal]

Hello,
  the documentation to AC_CHECK_PROG and friends says that
"if the variable is already set, the macro does nothing"

I propose to change it to something along the lines:

"if the variable is already set (and non-null), the macro does nothing"

BTW: have you seen my patches on address@hidden ?
One of the patches affects this part of manual, so you should apply it
first.

With kind regards,
        Stepan Kasal