OpenCOLLADA fails to build with GCC 6

GCC uses C++11 by default and this makes OpenCOLLADA fail the build with:

[ 70s] /home/abuild/rpmbuild/BUILD/OpenCOLLADA-3335ac164e68b2512a40914b14c74db260e6ff7d/COLLADABaseUtils/src/COLLADABUURI.cpp:57:2: error: narrowing conversion of '-1' from 'int' to 'char' inside { } [-Wnarrowing]

The code in question is:

    const char HEX2DEC[256] = 
    {
        /*       0  1  2  3   4  5  6  7   8  9  A  B   C  D  E  F */
        /* 0 */ -1,-1,-1,-1, -1,-1,-1,-1, -1,-1,-1,-1, -1,-1,-1,-1,

The 'issue' is that 'char' is not defined to be signed/unsigned (it depends on arch / implementation)
(i586/x86_64 use signed char, ppc64le and arm for example uses unsigned char)

It would likely be safe to change this to
const signed char HEX2DEC[256], this having all archs behave the same.

Start in COLLADABaseUtils/src/COLLADABUURI.cpp at the HEX2DEC declaration and reproduce the reported GCC 6 build error. Check the relevant build on GCC 6 and confirm that the change removes the narrowing error while preserving the existing URI conversion behavior across architectures.