Fix build on mingw32

fixed broken gettimeofday implementation for Windows

small, but critical typo :)

Fix and add test for Proc#source_location.

visualcpp toolchain: change compiler/linker flags

error.h: add extern "C" for C++

with these flags visualcpp should behave more like the gcc toolchain

c:
- produce debug information
- optimize for speed instead of disabling optimization
- use non-debug version of Microsoft's C library
- remove unused and unnecessary macro definitions
cxx:
- separate C and C++ (#1594)
- specify kind of exception handling (see warning C4530)
linker:
- produce debug information
- disable incremental linking and re-enable optimizations

__FILE__ test: add support for \ as separator

mruby-time: add gettimeofday(2) for Windows

mruby-time: various smaller changes

small refactor in mirb

Change the position of -L option from MRUBY_LIBS to MRUBY_LDFLAGS

remove unused variable

fix mruby command returning EXIT_SUCCESS when program file can't be opened.

GLOBAL_RAND_SEED_KEY is no longer used.

opened.

fix argument spec setting of Kernel.exit method.

this method requires 1 optional argument.

Add existing include/ directory only.

Fix #1803