Fix WebP support on MS-Windows

Fix WebP support on MS-Windows

* src/image.c (WebPDecodeRGBA, WebPDecodeRGB, WebPFree): Use
correct names and argument lists in DEF_DLL_FN; fix typos.
(WebPGetFeaturesInternal): Load this instead of WebPGetFeatures,
which is a static inline function in webp/decode.h.
(WebPGetFeatures): Redirect to call WebPGetFeaturesInternal.

* lisp/term/w32-win.el (dynamic-library-alist): Fix the name of
the WebP symbol.

* configure.ac (HAVE_WEBP): Fix detection of libwebp on MinGW.

* nt/INSTALL.W64:
* nt/INSTALL: Update information about libwebp availability.