lang/duktape-lib: add pkgconf file

This is needed for sysutils/polkit's duktape support, but also makes
it easier for other consumers to use the library.

While here, cosmetic cleanups.

PR:		254274
Reviewed by:	diizzy
Tested by:	vishwin
Approved by:	Alfonso S. Siciliano <alfix86[at]gmail[dot]com> (maintainer),
		fluffy (mentor)