v.builder: remove default link flags -lm, -ldl, -lpthread (use module specific link flags instead) (#10099)

vlib/sync/threads/threads.c.v

@@ -0,0 +1,13 @@
+module threads
+
+// This module adds the necessary compiler flags for using threads.
+// It is automatically imported by code that does `go func()` .
+// See vlib/v/parser/pratt.v, search for ast.GoExpr .
+// The goal is that programs, that do not use threads at all will not need
+// to link to -lpthread etc.
+// NB: on some platforms like Android, linking -lpthread is not needed too.
+// See https://stackoverflow.com/a/31277163/1904615
+
+$if !windows && !android {
+	#flag -lpthread
+}