Simplify secure API functions workaround for MinGW

If MINGW_HAS_SECURE_API is defined, the definitions of localtime_s,
fopen_s, _vsnprintf_s, strncpy_s had to be renamed using #define
so they would not conflict with definitions in the standard headers.

A simpler way to handle this is to undefine MINGW_HAS_SECURE_API before
including the standard headers.

Contributed by net147@gmail.com

BUGS=
TEST=

Review URL: https://chromiumcodereview.appspot.com/12210033
Patch from Jonathan Liu <net147@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13607 ce2b1a6d-e550-0410-aec6-3dcde31c8c00