Extract strcasecmp and friends to <bits/strcasecmp.h>.

Bug: http://b/28067717
Change-Id: I76bab098d4c35d07a7a108055c43c820087f9253
diff --git a/libc/include/strings.h b/libc/include/strings.h
index 1253006..1200e77 100644
--- a/libc/include/strings.h
+++ b/libc/include/strings.h
@@ -43,6 +43,8 @@
 #include <sys/cdefs.h>
 #include <xlocale.h>
 
+#include <bits/strcasecmp.h>
+
 __BEGIN_DECLS
 #if defined(__BIONIC_FORTIFY)
 #define bcopy(b1, b2, len) (void)(__builtin___memmove_chk((b2), (b1), (len), __bos0(b2)))
@@ -54,11 +56,6 @@
 
 int ffs(int);
 
-int strcasecmp(const char*, const char*) __purefunc;
-int strcasecmp_l(const char*, const char*, locale_t) __purefunc;
-int strncasecmp(const char*, const char*, size_t) __purefunc;
-int strncasecmp_l(const char*, const char*, size_t, locale_t) __purefunc;
-
 __END_DECLS
 
 #endif /* !defined(_STRINGS_H_) */