build: Include <strings.h>; add strcasecmp() check to configure.

strings.h (not string.h) is the standard-defined include header for
strcasecmp(). Include <strings.h> for portability (even though glibc
exposes strcasecmp() declaration also in <string.h> by default).

configure.ac

@@ -111,7 +111,7 @@ AC_PATH_PROG([INDENT], indent, [\${top_srcdir}/build-aux/missing indent])
 
 # checks for headers
 
-AC_CHECK_HEADERS([regex.h sys/stat.h sys/wait.h unistd.h], [],
+AC_CHECK_HEADERS([regex.h strings.h sys/stat.h sys/wait.h unistd.h], [],
   [AC_MSG_ERROR(required header not found on your system)])
 
 AC_CHECK_HEADERS([inttypes.h libintl.h limits.h locale.h malloc.h netinet/in.h])
@@ -153,6 +153,7 @@ AS_IF([test "$cross_compiling" = yes],
 AC_CHECK_FUNCS([dup2 dnl
 memset dnl
 regcomp dnl
+strcasecmp dnl
 strchr dnl
 strdup dnl
 strtol dnl

src/flexdef.h

@@ -73,6 +73,8 @@
 #include <stdarg.h>
 /* Required: regcomp(), regexec() and regerror() in <regex.h> */
 #include <regex.h>
+/* Required: strcasecmp() in <strings.h> */
+#include <strings.h>
 #include "flexint.h"
 
 /* We use gettext. So, when we write strings which should be translated, we mark them with _() */