Update to 2.4.49:

A new version of libdrm has been released. It adds support for 2D
tiling for Sea Islands Radeon GPUs and fixes bugs.

Ian Romanick (1):
      intel: Use memset instead of VG_CLEAR

Marek Olšák (3):
      radeon: implement 2D tiling for CIK
      radeon: handle P16 pipe configs for Hawaii
      Bump the version for release

Michel Dänzer (2):
      radeon: fix mipmap level 0 and 1 alignment for SI and CIK
      radeon: don't overallocate stencil by 4 on SI and CIK

Thomas Hellstrom (1):
      libdrm/mode: Update the encoder and connector defines
Updated x11/libdrm to 2.4.49
fixes 3rdparty header path
Set Interix bultin gcc specific patths to COMPILER_{INCLUDE,LIB}_DIRS.
Introduce find-headers.mk, ablility to find builtin header files.
Inspired by find-libs.mk and find-files.mk.
Use find-headers instead of find-files to detect builtin header files.
Use find-headers instead of find-files to detect builtin header files.
Use xcrun to determine the full path to the SDK, and use the headers from
that location in the new find-headers infrastructure when they have not
been installed into /usr/include.

This allows us to remove the hardcoded builtins, as they can now be
correctly determined automatically.