If CMake is run and Doxygen is found, add a make target called "doc",
which builds API documention in doc/html. This target is not included
in the default "all" target, you have to explicitly run "make doc" to
generate the documentation.

If graphviz is installed, in particular, if the "dot" binary is found,
doxygen is configured to generate various kinds of diagrams. Note that
due to this, the first run of doxygen can take a while.

Allow footstep sounds to play for liquid and ladder nodes, making swimming and climbing sounds possible

Signed-off-by: Craig Robbins <kde.psych@gmail.com>

LuaPerlinNoiseMap: Prevent invalid memory access when attempting to generate 3d noise with a buffer created for 2d

This reverts commit 9878e8de.

See: https://github.com/minetest/minetest/issues/1939 and IRC log for discussion

This greatly reduces the complexity of Settings code.
Additionally, several memory leaks were fixed.

Signed-off-by: Craig Robbins <kde.psych@gmail.com>

Leads to the following increases:
getSmoothLight() approx.     40% increase
getTileInfo() approx.        25% increase
MapBlockMesh::MapBlockMesh() 25-30%

Details:
- https://sourceforge.net/p/irrlicht/bugs/433/
- https://github.com/minetest/minetest/issues/1687#issuecomment-61368769
- https://forum.minetest.net/viewtopic.php?f=42&t=9190&start=125#p159364

In case when "settings.h" is included from "emerge.cpp" or
"environment.cpp", u64 type has "unsigned long" length because
previously <stdint> was included. When "settings.h" is included from
"settings.cpp", u64 has "unsigned long long" length because no <stdint>
was included previously. This leads to different signatures of "setU64" method
and linker cannot find appropriate symbol.

The best fix of this bug should be done in the Irrlicht, but as hotfix I
think this is OK and better than types changing.

Previously this bug didn't appear because there was no "settings.cpp" file and
implementation of all methods was done in the header file.

Add 'absolute value' option to noise map functions
Extend persistence modulation to 3D noise
Extend 'eased' option to noise2d_perlin* functions
Some noise.cpp formatting fixups