This complements the work done in d4928ed, allowing the user to specify
the preprocessor flags with the CPPFLAGS environment variable. This is
useful for example to specify preprocessor macros with -D.

CFLAGS could be used instead, but CPPFLAGS is more correct and is expected
to be honored in some cases. For example, the helper scripts to build
Debian packages make use of CPPFLAGS, but the variable is currently
being ignored unless manually appended to CFLAGS.

In this way the path of pkg-config can be overridden from the command
line. This is useful for example when cross-compiling.

Change some styling too while we're at it.

strsep() is not a POSIX function, and it means that every system
needs different defines to expose it. If the prototype of strsep
is not exposed then an ugly int/pointer is done and it might mean
a crash. The best solution?, to remove the strsep and make a custom
loop. If C programmers cannot do this kind of loops without calling
a library function, then maybe we should move all the suckless
software to Java.

CC by default is cc, so the assignment was doing nothing, but
it was using non standard syntax, so some system (NetBSD) fail
to compile.

The clock_gettime patch needs librt to be linked to surf.

Thanks _odie for the hint.

This macro was not correct in some cases, and it was used only in
one place, where we did'nt get any benefit in performance of in size,
so the macro is removed and ceilf is used instead of it. The only
function needed from math.h is ceilf, so this patch defines the
prototype of it instead of including math.h.

Thanks for the hint from Alexander Sedov <alex0player@gmail.com>!

invert mode now works as expected. In the config.def.h autohint is set to
false, so the fonts are drawn correctly, without any overlapping.

shift+insert paste clipboard
honor CFLAGS and LDFLAGS in config.mk

started without a display. (thx Hiltjo Posthuma)

Due to consideration of POSIX compliance issues config.mk had
to be removed. Configuration variables can be overridden by
environment variables or specified via command line.

Additionally all pretty-printed messages were removed and
built-in rules are used. This also simplifies and purges
the Makefile.

The xinerama related variables in config.mk are a relict of
dwm and therefore should be removed.