ncurses: install *.pc files (for pkg-config)

To make e.g. "pkg-config --cflags ncursesw" work.

The ncurses expression, when built in unicode/wide char mode, provides
backward compatibility symlinks from lib<name>.so to lib<name>w.so.
Provide similar symlinks for the *.pc files: <name>.pc -> <name>w.pc.
ncurses: add ncurses5-config -> ncursesw5-config symlink

We already have backward compatibility symlinks for ncurses libraries
and pkg-config files (when built in unicode/wide mode). The last bit is
to add a symlink from ncurses5-config to ncursesw5-config, which is what
this commit does.
Merge pull request #1408 from bjornfor/ncurses-enable-pc-files

ncurses: install *.pc files (pkg-config)