i#2522 strcasestr prototype error: Remove const from result type

The definition of strcasestr is

char *strcasestr(const char *text, const char *pattern);

but drmemory uses

const char *strcasestr(const char *text, const char *pattern);

This causes build errors when both /usr/include/string.h and common/utils.h
are included in linux. The definition for strcasestr for C has always not
had the `const` in the result type (going back to its original addition
to glibc in 1997). Thus we're pretty safe in not breaking anything on *nix.
The other main use case of drmemory's strcasestr is on Windows, which does
not have strcasestr.

Tested:
$ cmake && make

Fixes #2522

common/utils.h

@@ -1011,7 +1011,7 @@ drmem_strndup(const char *src, size_t max, heapstat_t type);
 #define MAX_OPTION_LEN DR_MAX_OPTIONS_LENGTH
 
 #if !defined(MACOS) && !defined(ANDROID) && !defined(NOLINK_STRCASESTR)
-const char *
+char *
 strcasestr(const char *text, const char *pattern);
 #endif
 

common/utils_shared.c

@@ -51,7 +51,7 @@ strnchr(const char *str, int find, size_t max)
  * want a libc dependence.
  */
 #if !defined(MACOS) && !defined(ANDROID) && !defined(NOLINK_STRCASESTR)
-const char *
+char *
 strcasestr(const char *text, const char *pattern)
 {
     const char *cur_text, *cur_pattern, *root;
@@ -60,7 +60,7 @@ strcasestr(const char *text, const char *pattern)
     cur_pattern = pattern;
     while (true) {
         if (*cur_pattern == '\0')
-            return root;
+            return (char *) root;
         if (*cur_text == '\0')
             return NULL;
         /* XXX DRi#943: toupper is better, for int18n, and we need to call