configure: Rename the recently-added options

Rename
    --enable-png-tests/--disable-png-tests
    --enable-png-tools/--disable-png-tools
to
    --enable-tests/--disable-tests
    --enable-tools/--disable-tools
respectively.

While the `PNG_` prefix is necessary in CMakeLists.txt, a corresponding
`--...-png-` option qualifier would be redundant in a configure script.
2 files changed