Load libgccjit dynamically in Windows.

Load libgccjit dynamically in Windows.

* configure.ac: don't add linker flags if compiling on
Windows. Compile dynlib.c if modules or native compilation are
enabled. Always compile comp.c
* lisp/term/w32-win.el: Map 'gccjit to "libgccjit.dll" in
`dynamic-library-alist`.
* src/Makefile.in: Update comments. Update to handle changes in
configure.ac.
* src/comp.c: Add declarations of used libgccjit functions using
DEF_DLL_FN. Add calls to load_gccjit_if_necessary() where necessary.
Add `native-comp-available-p`
* src/comp.h: Remove Fnative_elisp_load. Add syms_of_comp().
* src/emacs.c (main): Always call syms_of_comp()
* src/w32.c (globals_of_w32): Clear Vlibrary_cache when starting
because the libraries loaded when dumping will not be loaded when
starting.
* src/w32fns.c: Add Qgccjit symbol.