MSYS build scripts and instructions (#99) and warning fixes

* Preliminary MSYS2 + MinGW build scripts and instructions.

* CMakeLists.txt: MinGW tweaks, enforce C++03, suppress warnings for now.

* Fix some GCC warnings.

* Fix unused parameter warnings coming from Ifc*.h.

* Fix warning regarding unreachable code (++jt) on MSVC.

* Add IfcParse_EXPORT for IfcInvalidTokenException

* Fix potentially uninitialized pointer variables.

* Note about OpenCASCADE cyclic dependencies fix.

* GCC warning fix: don't generate 'current_enum' variable that is only set and not used for anything ever

* Work around -Wmaybe-uninitialized warnings about boost::optional constructs. Also prevent passing of negative values for --bounds.

* Remove unused variable.

* CMakeLists.txt: ENABLE_BUILD_OPTIMIZATIONS for GCC (simply enforce -03)