* winnt.h (C_ASSERT): Revert the EXTERN_C change from rev.4285/4286.
* usbioctl.h: Remove the extern "C" {} from the header completely.
There are no functions or variables are declared, only data types,
therefore it is not needed.
* dbghelp.h, imagehlp.h: move the extern "C" braces so that the
C_ASSERTs are not contained within them. (bug #3373905.)
* crtdbg.h, malloc.h (_STATIC_ASSERT): Restrict the typedef variant
of the macro to MSVC. Add an extern declaration variant for gcc,
similar to C_ASSERT.

git-svn-id: svn+ssh://svn.code.sf.net/p/mingw-w64/code/branches/releases/v1.0@4292 4407c894-4637-0410-b4f5-ada5f102cad1