Update embeding flags

* Include threading flags. Fix #16868
* Add document about exporting symbols from main executable. Fix #17451

contrib/julia-config.jl

@@ -8,6 +8,14 @@ const options =
     "--ldlibs"
 ];
 
+function threadingOn()
+    try
+        return cglobal(:jl_tls_states) == C_NULL
+    catch
+        return true
+    end
+end
+
 function imagePath()
     opts = Base.JLOptions()
     unsafe_string(opts.image_file)
@@ -57,10 +65,11 @@ end
 function cflags()
     arg1 = replace(initDir(),"\\","\\\\\\\\")
     arg2 = replace(includeDir(),"\\","\\\\")
+    threading_def = threadingOn() ? "-DJULIA_ENABLE_THREADING=1 " : ""
     if is_unix()
-        return """-fPIC -DJULIA_INIT_DIR=\\"$arg1\\" -I$arg2"""
+        return """$(threading_def)-fPIC -DJULIA_INIT_DIR=\\"$arg1\\" -I$arg2"""
     else
-        return """-DJULIA_INIT_DIR=\\"$arg1\\" -I$arg2"""
+        return """$(threading_def)-DJULIA_INIT_DIR=\\"$arg1\\" -I$arg2"""
     end
 end
 

doc/manual/embedding.rst

@@ -56,6 +56,10 @@ Before the program terminates, it is strongly recommended to call ``jl_atexit_ho
    >>> julia.jl_init('.')
    250593296
 
+.. note::
+
+ If the julia program needs to access symbols from the main executable, it may be necessary to add ``-Wl,--export-dynamic`` linker flag at compile time on Linux in additional to the ones generated by ``julia-config.jl`` described below. This is not necessary when compiling a shared library.
+
 Using julia-config to automatically determine build parameters
 --------------------------------------------------------------