Allow overriding build tools from configure command line

Introduce GP_CHECK_PROG macro to allow users to override
the autodetection of tools, instead of the autodetection
overriding the user's explicit wish.

    GP_CHECK_PROG([SORT, [sort], [text line sorting tool])

will now allow calling

    ./configure SORT="mysort"

and have subsequent builds use that given value without
checking PATH. When calling `configure` without SORT=,
AC_PATH_PROG will look for `sort` in `PATH` and set
`SORT` to the value it has found.

For comparison, here is the old buggy method:

    AC_ARG_VAR([SORT], [sort], [text line sorting tool])
    AC_PATH_PROG([SORT], [sort], [no])

When called as `configure SORT="mysort"`, the expanded
AC_PATH_PROG will just look in `PATH` for `sort` and
set `SORT` to the `sort` it found, completely ignoring
the SORT= value given on the `configure` command line.

0 files changed, 0 insertions, 0 deletions