long double for windows using mingw-w64

The following pull request provides long double type (extflonum) on
win32.

Extflonum arithmetic is implemented in a set of functions
compiled into longdouble.dll (dll and lib attached, source
code included in the pull request). It has been compiled
with mingw-w64. In ordinary mingw there is no long double
input (neither non-msvc strtold nor scanf).