document ZSTD_USE_C90_QSORT

Author: Cyan4973

lib/README.md

@@ -193,6 +193,10 @@ The file structure is designed to make this selection manually achievable for an
   and assembly decoding loops. You may want to use this macro if these loops are
   slower on your platform.
 
+- The macro `ZSTD_USE_C90_QSORT` forces usage of C90's `qsort()`,
+  for situations where the code cannot determine that `qsort_r()` is not supported,
+  such as, for example, older versions of `musl`.
+
 #### Windows : using MinGW+MSYS to create DLL
 
 DLL can be created using MinGW+MSYS with the `make libzstd` command.