Workaround an include loop on case-insensitive systems

On case-insensitive systems, #include <exception> includes our own
fancy header "Exception" instead of the stdlib exception header.
To disambiguate this, we resolve a full absolute path to the stdlib
header (through some magic of parsing compiler debug output) and
generate std_exception.h include file that we then use in our
excepton.h to include the stdlib exception header.