Introduce _OPSYS_LIB_DIRS and _OPSYS_INCLUDE_DIRS varables.
list of system library and header file directories.

Also add missing _OPSYS_SYSTEM_RPATH for some platforms.
Introduce COMPILER_INCLUDE_DIRS, COMPILER_LIB_DIRS and SYSTEM_DEFAULT_RPATH.
default value are _OPSYS_INCLUDE_DIRS, _OPSYS_LIB_DIRS and _OPSYS_DEFAULT_RPATH,
defined in mk/platform/${OPSYS}.mk.
(maybe defined with compiler/development tools specific variables).
* use SYSTEM_DEFAULT_RPATH/COMPILER_INCLUDE_DIRS/COMPILER_LIB_DIRS
  instead of hard-coded /usr/include, /usr/lib, ... paths.
* allow empty BUILDLINK_PREFIX.${_pkg_}, for builtin packags not match such
  model (Haiku's system headers and libraries are in different hier).
Let to find libraires in COMPILER_LIB_DIRS instead of hard coded paths.
tell pthread of Haiku.
Fixes builtin dl prefix other than /usr.
Added math/gnumeric112 version 1.12.0 [drochner 2013-03-14]
+ blender-2.67, dbus-1.7.2, galculator-2.1.1, gnumeric112-1.12.2,
  mono-3.0.10, parallel-20130422, py-icalendar-3.4, qemu-1.4.1,
  x264-devel-20130427.