identify all of the available task context creation algorithms with HAVE_ flags for better clarity and simplicity

Author: vtjnash

src/julia.h

@@ -1386,13 +1386,10 @@ typedef struct _jl_task_t {
     // hidden state:
 #ifdef _OS_WINDOWS_
     LPVOID fiber; // Fiber that this runs on
-    jl_jmp_buf ctx; // saved thread state
-#else
-    unw_context_t ctx; // saved unwind context
 #endif
+    jl_jmp_buf ctx; // saved thread state
     void *stkbuf; // malloc'd memory
-    unsigned ssize; // sizeof the portion of stack used in stkbuf
-    uint8_t started;
+    int ssize; // sizeof the portion of stack used in stkbuf
 
     // current exception handler
     jl_handler_t *eh;

src/options.h

@@ -87,10 +87,8 @@
 
 // task options ---------------------------------------------------------------
 
-// select an implementation of stack switching.
-// currently only COPY_STACKS is recommended.
+// select whether to enable the COPY_STACKS stack switching optimization
 #define COPY_STACKS
-
 #define JL_STACK_SIZE (8*1024*1024)
 
 // sanitizer defaults ---------------------------------------------------------