Add a ".bash" extension to the main include script make explicit the
intent for it to be used with Bash.

Like we did for the `incdec.m4` Bash include, rename the Perl M4 file to
give it a `.pl` extension so we get Perl syntax highlighting. Similarly
define an include macro so quoting doesn't mess up highlighting.

Put the file after the option argument since it's more common to order
them that way.

Put build targets above the test target.

When I changed the file to an M4 file, it removed Bash syntax
highlighting, and the `include()` call interfered with Bash quote
matching. Use a different macro to include the Perl file so we can get
Bash syntax highlighting.

Include the built `incdec.pl` code in the Bash file by generating it
from `incdec.pm`.

Build an intermediary Perl file that includes the extra lines that call
`incdec()`. This whole file should be included in the final Bash
function `perl -e` call.