Updates for MSVC

patch utf8proc so it can compile in either C or C++ mode

dont define USE_COMPUTED_GOTO for MSVC
(might be an odd double-definition problem from some header)

add utf8proc info to src/flisp/Windows.mk
(needs some improvement on handling of utf8proc
version number and assumption of cl vs icl)

add strtod to Windows.mk

add DLLEXPORT before jl_init_frontend(void)