Allow optimized builds

[sloede]

Right now, we build the KROME library with the standard settings, that is, no optimizations are enabled at compile time but the compiler's default optimization level is used. We should support also building the optimized versions (possibly controlled by an environment variable, e.g., JULIA_KROME_OPTIMIZED_BUILD=(yes|1|true)). This would become much easier once the Add make target to build shared library and C interface issue in #7 is resolved.