Fix compilation warnings in MinGW64 build using GCC 7

Fix compilation warnings in MinGW64 build using GCC 7

Reported by Richard Copley <rcopley@gmail.com>.
* src/w32heap.c (init_heap): Declare enable_lfh only for
mingw.org's MinGW build.

* src/w32console.c (w32con_write_glyphs):
* src/unexw32.c (get_section_info, COPY_CHUNK, unexec): Fix some
mismatches of data type vs format spec.

* src/w32fns.c (compute_tip_xy):
* src/w32proc.c (stop_timer_thread):
* src/w32notify.c (remove_watch):
* src/eval.c (internal_lisp_condition_case):
* src/editfns.c (Ftranslate_region_internal):
* src/data.c (Fmake_variable_buffer_local, cons_to_unsigned)
(cons_to_signed):
* src/buffer.c (fix_overlays_before): Initialize variables to
avoid compiler warnings.

* lib-src/etags.c (TeX_commands, process_file_name): Initialize
variables to avoid compilation warnings.