#8 Fix endian handling

[Ryan Carsten Schmidt]

Compiling DevIL 1.7.8 fails on OS X 10.9 Mavericks:

libtool: compile:  ccache /usr/bin/clang++ -DHAVE_CONFIG_H -I. -I../include/IL -I ./../src-IL/include -I ./../include -isystem/opt/local/include -fgnu89-inline -msse -msse2 -msse3 -pipe -Os -arch x86_64 -MT libIL_la-il_utx.lo -MD -MP -MF .deps/libIL_la-il_utx.Tpo -c ./../src-IL/src/il_utx.cpp  -fno-common -DPIC -o .libs/libIL_la-il_utx.o
In file included from ./../src-IL/src/il_utx.cpp:17:
In file included from ./../src-IL/include/il_utx.h:31:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/memory:588:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__config:31:22: error: expected value in expression
#if __LITTLE_ENDIAN__
                     ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__config:86:11: fatal error: 'endian.h' file not found
# include <endian.h>
          ^
2 errors generated.
make[1]: *** [libIL_la-il_utx.lo] Error 1

This is because the system headers expect that if __LITTLE_ENDIAN__ is defined, then it is defined to a truthy value, and this DevIL header breaks that assumption by undefining it and then redefining it as empty. The fix is to remove the lines that undefine and redefine __LITTLE_ENDIAN__. Since there were already lines defining __BIG_ENDIAN__ to 1 on big-endian systems if undefined, I added lines to do the same for __LITTLE_ENDIAN__ on little-endian systems, though they're not needed to build successfully on OS X because OS X already defines __LITTLE_ENDIAN__ to 1.

While looking into fixing this, I noticed that DevIL will never correctly detect big-endian systems because it compares __BYTE_ORDER__ against __BIG_ENDIAN__, which is the wrong constant. The fix is to compare against __ORDER_BIG_ENDIAN__.

I fixed this in MacPorts with this patch:

https://trac.macports.org/browser/trunk/dports/devel/libdevil/files/patch-src-IL-include-il_endian.h.diff?rev=112877

ResIL 1.7.9 does not have the same build failure, but applying the same patch to ResIL nonetheless seems correct, at least to correct the detection of big-endian systems.