Issues/0134 openexr2 shared iex_debug trap

[michaeldsmith]

• adds OpenEXR target link libraries to CMakeLists.txt as recommended by OpenEXR Porting Guide https://openexr.com/en/latest/PortingGuide.html
• fixes undefined symbols issue described in #134 that seems to occur when building OpenEXR and CTL using shared libraries, -DBUILD_SHARED_LIBS=ON

1. [ 68%] Building CXX object ctlrender/CMakeFiles/ctlrender.dir/format.cc.o
   [ 69%] Building CXX object ctlrender/CMakeFiles/ctlrender.dir/compression.cc.o
   [ 70%] Linking CXX executable ctlrender
   ld: Undefined symbols:
   half::_eLut, referenced from:
     half::half(float) in exr_file.cc.o
   half::convert(int), referenced from:
     half::half(float) in exr_file.cc.o
   half::_toFloat, referenced from:
     half::operator float() const in libIlmCtlSimd.a[14](CtlSimdStdLibLookupTable.cpp.o)

2. [ 23%] Building CXX object lib/IlmCtlMath/CMakeFiles/IlmCtlMath.dir/CtlRbfInterpolator.cpp.o
   [ 24%] Linking CXX shared library libIlmCtlMath.dylib
   Undefined symbols for architecture x86_64:
   "iex_debugTrap()", referenced from:
     double Ctl::CG<double, Ctl::LSSOperator<double, Ctl::CRSOperator<double> >, Ctl::NullLinearOperator>::operator()<std::__1::__wrap_iter<double*>, std::__1::__wrap_iter<double*> >(std::__1::__wrap_iter<double*>, std::__1::__wrap_iter<double*>, std::__1::__wrap_iter<double*>, std::__1::__wrap_iter<double*>) const in CtlRbfInterpolator.cpp.o
   "Iex_2_5::ArgExc::ArgExc(std::__1::basic_stringstream<char, std::__1::char_traits<char>, std::__1::allocator<char> >&)", referenced from:
     double Ctl::CG<double, Ctl::LSSOperator<double, Ctl::CRSOperator<double> >, Ctl::NullLinearOperator>::operator()<std::__1::__wrap_iter<double*>, std::__1::__wrap_iter<double*> >(std::__1::__wrap_iter<double*>, std::__1::__wrap_iter<double*>, std::__1::__wrap_iter<double*>, std::__1::__wrap_iter<double*>) const in CtlRbfInterpolator.cpp.o