Port --enable-gcc-warnings to GCC 7

Do not merge to master.
* configure.ac: Pacify GCC 7 with -Wno-bool-operation
-Wno-implicit-fallthrough.
* src/search.c (boyer_moore):
Tell GCC that CHAR_BASE, if nonzero, must be a non-ASCII character.
* src/xterm.c (x_draw_glyphless_glyph_string_foreground):
Tell GCC that glyph->u.glyphless.ch must be a character.

configure.ac

@@ -953,6 +953,8 @@ AS_IF([test "$gl_gcc_warnings" != yes],
     gl_WARN_ADD([$w])
   done
   gl_WARN_ADD([-Wredundant-decls])     # Prefer this, as we don't use Bison.
+  gl_WARN_ADD([-Wno-bool-operation])   # Emacs 26 is clean, 25 is not.
+  gl_WARN_ADD([-Wno-implicit-fallthrough]) # Emacs 26 is clean, 25 is not.
   gl_WARN_ADD([-Wno-missing-field-initializers]) # We need this one
   gl_WARN_ADD([-Wno-sign-compare])     # Too many warnings for now
   gl_WARN_ADD([-Wno-type-limits])      # Too many warnings for now

src/search.c

@@ -1833,6 +1833,7 @@ boyer_moore (EMACS_INT n, unsigned char *base_pat,
     {
       /* Setup translate_prev_byte1/2/3/4 from CHAR_BASE.  Only a
 	 byte following them are the target of translation.  */
+      eassume (0x80 <= char_base && char_base <= MAX_CHAR);
       unsigned char str[MAX_MULTIBYTE_LENGTH];
       int cblen = CHAR_STRING (char_base, str);
 

src/xterm.c

@@ -1911,6 +1911,7 @@ x_draw_glyphless_glyph_string_foreground (struct glyph_string *s)
 	}
       else if (glyph->u.glyphless.method == GLYPHLESS_DISPLAY_HEX_CODE)
 	{
+	  eassume (glyph->u.glyphless.ch <= MAX_CHAR);
 	  sprintf (buf, "%0*X",
 		   glyph->u.glyphless.ch < 0x10000 ? 4 : 6,
 		   glyph->u.glyphless.ch + 0u);