wmifinfo: Update Makefile.

In particular, add support for common user-defined variables CFLAGS,
CPPFLAGS, LDFLAGS, DESTDIR, and PREFIX.  Also remove reference to
deprecated X11R6 directory and use install instead of cp for install
target.

Inspired by the Debian patches:
http://sources.debian.net/src/wmifinfo/0.09-7/debian/patches/makefile_add_ldflags.patch/
http://sources.debian.net/src/wmifinfo/0.09-7/debian/patches/makefile_patch.patch/
wmifinfo: Add manpage.

Adapted from Debian manpage:
http://sources.debian.net/src/wmifinfo/0.09-7/debian/wmifinfo.1/
wmifinfo: Fix -Wpointer-sign compiler warnings.
wmifinfo: Fix -Wunused-but-set-variable compiler warning.
wmifinfo: Fix -Wunused-result compiler warnings.
wmifinfo: Remove C++ style comments.
wmifinfo: Fix -Wpedantic compiler warning.
wmifinfo: Fix -Wformat compiler warnings.
wmifinfo: Add valid timeval.

Patch by Julien BLACHE from Debian:
http://sources.debian.net/src/wmifinfo/0.09-7/debian/patches/wmifinfo-timeval_fix.patch/

For more information, see:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=503416
wmifinfo: Bump to version 0.10.