Fix build on big endian architectures.

On big endian architectures, the build fails with the following error:

    In file included from /build/buildd-minetest_0.4.6+repack-3-powerpc-92VoFb/minetest-0.4.6+repack/src/cguittfont/xCGUITTFont.h:6:0,
                     from /build/buildd-minetest_0.4.6+repack-3-powerpc-92VoFb/minetest-0.4.6+repack/src/cguittfont/xCGUITTFont.cpp:4:
    /build/buildd-minetest_0.4.6+repack-3-powerpc-92VoFb/minetest-0.4.6+repack/src/cguittfont/irrUString.h:805:19: error: #if with no expression
    /build/buildd-minetest_0.4.6+repack-3-powerpc-92VoFb/minetest-0.4.6+repack/src/cguittfont/irrUString.h:819:19: error: #if with no expression
    /build/buildd-minetest_0.4.6+repack-3-powerpc-92VoFb/minetest-0.4.6+repack/src/cguittfont/irrUString.h:833:19: error: #if with no expression
    /build/buildd-minetest_0.4.6+repack-3-powerpc-92VoFb/minetest-0.4.6+repack/src/cguittfont/irrUString.h:848:19: error: #if with no expression
    /build/buildd-minetest_0.4.6+repack-3-powerpc-92VoFb/minetest-0.4.6+repack/src/cguittfont/irrUString.h:862:19: error: #if with no expression
    /build/buildd-minetest_0.4.6+repack-3-powerpc-92VoFb/minetest-0.4.6+repack/src/cguittfont/irrUString.h:881:19: error: #if with no expression
    /build/buildd-minetest_0.4.6+repack-3-powerpc-92VoFb/minetest-0.4.6+repack/src/cguittfont/irrUString.h:896:19: error: #if with no expression
    /build/buildd-minetest_0.4.6+repack-3-powerpc-92VoFb/minetest-0.4.6+repack/src/cguittfont/irrUString.h:911:19: error: #if with no expression
    /build/buildd-minetest_0.4.6+repack-3-powerpc-92VoFb/minetest-0.4.6+repack/src/cguittfont/irrUString.h:925:19: error: #if with no expression
    /build/buildd-minetest_0.4.6+repack-3-powerpc-92VoFb/minetest-0.4.6+repack/src/cguittfont/irrUString.h:939:19: error: #if with no expression
    /build/buildd-minetest_0.4.6+repack-3-powerpc-92VoFb/minetest-0.4.6+repack/src/cguittfont/irrUString.h:953:19: error: #if with no expression
    /build/buildd-minetest_0.4.6+repack-3-powerpc-92VoFb/minetest-0.4.6+repack/src/cguittfont/irrUString.h:967:19: error: #if with no expression
    /build/buildd-minetest_0.4.6+repack-3-powerpc-92VoFb/minetest-0.4.6+repack/src/cguittfont/irrUString.h:981:19: error: #if with no expression
    /build/buildd-minetest_0.4.6+repack-3-powerpc-92VoFb/minetest-0.4.6+repack/src/cguittfont/irrUString.h:995:19: error: #if with no expression
    /build/buildd-minetest_0.4.6+repack-3-powerpc-92VoFb/minetest-0.4.6+repack/src/cguittfont/irrUString.h:1009:19: error: #if with no expression
    /build/buildd-minetest_0.4.6+repack-3-powerpc-92VoFb/minetest-0.4.6+repack/src/cguittfont/irrUString.h:1028:19: error: #if with no expression

(Full build log at:https://buildd.debian.org/status/fetch.php?pkg=minetest&arch=powerpc&ver=0.4.6%2Brepack-3&stamp=1366277397 )