convert some warnings to errors

rework Makefile

- allow user to use a different pkg-config binary
- restore almost all (and add other ones) warning flags (-Werror is not set)
- $(XWAYLAND) is added to our CPPFLAGS
- remove useless comments
- don't generate idle-protocol.h (not used)

define _POSIX_C_SOURCE through CPPFLAGS

like dwm

fix dist target

include DWLCFLAGS into linking step

Merge pull request #251 from gwnichol/fixinstall

Fix make install while files are in use

Fix make install while files are in use

When the dwl executable is in use, cp fails without the -f flag.

POSIX defines this flag with:
> If a file descriptor for a destination file cannot be obtained,
> as described in step 3.a.ii., attempt to unlink the destination
> file and proceed.

add $(LIBS) to $(LDLIBS)

fix link failure with --as-needed

use LDFLAGS to build dwl target

do not generate *-protocol.{c,o}

according with
https://gitlab.freedesktop.org/wlroots/wlroots/-/commit/62fbf3f4ba1b2993e29dfb46f077e5806f7aac1c
they are unused and wlroots-based compositors don't need to do this

include xcb only when building with xwayland support

convert makefile to be more portable

add dist target

add flag to print version and exit

add DESTDIR

don't let -pedantic be overridable by environment

new functions ecalloc() and die()

die() replaces EBARF and BARF
and allow us to add `-pedantic` to CFLAGS

Merge pull request #180 from Humm42/manpage

add dwl(1)

add dwl(1)

Documentation is good.  Man pages are documentation.  A program without
a man page is worthless.