getopt: Fix conversion from string-literal to non-const char *

Define a non-const static char EMSG[] = "" to avoid having to add
__DECONST() to all uses of EMSG. Also make current_dash a const char *
to fix this warning.

lib/libc/stdlib/getopt.c

@@ -54,8 +54,7 @@ char	*optarg;		/* argument associated with option */
 
 #define	BADCH	(int)'?'
 #define	BADARG	(int)':'
-#define	EMSG	""
-
+static char EMSG[] = "";
 /*
  * getopt --
  *	Parse argc/argv argument vector.

lib/libc/stdlib/getopt_long.c

@@ -88,7 +88,7 @@ char    *optarg;		/* argument associated with option */
 #define	BADARG		((*options == ':') ? (int)':' : (int)'?')
 #define	INORDER 	(int)1
 
-#define	EMSG		""
+static char EMSG[] = "";
 
 #ifdef GNU_COMPATIBLE
 #define NO_PREFIX	(-1)
@@ -194,7 +194,7 @@ parse_long_options(char * const *nargv, const char *options,
 {
 	char *current_argv, *has_equal;
 #ifdef GNU_COMPATIBLE
-	char *current_dash;
+	const char *current_dash;
 #endif
 	size_t current_argv_len;
 	int i, match, exact_match, second_partial_match;

tools/build/mk/Makefile.boot

@@ -41,6 +41,7 @@ CWARNFLAGS+=	-Wno-typedef-redefinition
 # bsd.sys.mk explicitly turns on -Wsystem-headers, but that's extremely
 # noisy when building on Linux.
 CWARNFLAGS+=	-Wno-system-headers
+CWARNFLAGS.clang+=-Werror=incompatible-pointer-types-discards-qualifiers
 
 # b64_pton and b64_ntop is in libresolv on MacOS and Linux:
 # TODO: only needed for uuencode and uudecode