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

Bug: http://b/28067717
Change-Id: I76bab098d4c35d07a7a108055c43c820087f9253
diff --git a/libc/include/string.h b/libc/include/string.h
index 32d4a18..dada52b 100644
--- a/libc/include/string.h
+++ b/libc/include/string.h
@@ -33,6 +33,8 @@
 #include <stddef.h>
 #include <xlocale.h>
 
+#include <bits/strcasecmp.h>
+
 __BEGIN_DECLS
 
 #if defined(__USE_BSD)
@@ -72,11 +74,6 @@
 extern char*  strcpy(char* __restrict, const char* __restrict);
 extern char*  strcat(char* __restrict, const char* __restrict);
 
-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;
-
 extern char*  strdup(const char *);
 
 extern char*  strstr(const char *, const char *) __purefunc;