Improve Makefile

- Change compiling flag -Wpedantic to -pedantic.
  Flag -Wpedantic is not available in older versions of gcc,
  what makes the life of some users unnecessarily harder.
  Not to mention that both flags work fine for F3.

- Allow make variable CC to be set externally.
  Some platforms favor other C compilers.
  For example, FreeBSD defaults to clang.

- Allow make variable CFLAGS to be extended externally.
  This helps packaging.

- Target clean to also remove *.d files.

The three first improvements were suggested by
Thomas Fischer and Uffe Jakobsen.
The discussion is available here:

https://github.com/AltraMayor/f3/issues/4
Bump next version to 4.0

Given that support for FreeBSD was added,
the next release will no longer be 3.1, but 4.0.