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.

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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.