Avoid compilation warnings with GCC 7 on MS-Windows

Avoid compilation warnings with GCC 7 on MS-Windows

* src/w32term.c (w32_setup_relief_color, construct_mouse_click)
(w32_read_socket): Initialize variables to shut up bogus
compilation warnings from GCC 7.
* src/unexw32.c (COPY_CHUNK, COPY_PROC_CHUNK): Cast to DWORD_PTR
to avoid compiler warnings about printing signed values using %x
format spec.
* src/dispnew.c (adjust_glyph_matrix): Add eassert to avoid
compiler warning about possible NULL pointer dereference.
* src/lisp.h (pI): Tweak the definition some more for MinGW64.