Add -lc and -pthread to shared library builds

The option -nostdlib that is used for shared library builds also
removes -lc and -pthread. While this does not immediately cause problems
when building the shared version of the runtime it won't use the correct
symbol versions causing it to use the wrong version of some libc
symbols.

ref eng/cert/certified-rts#145

support/data/target_options.gpr.in

@@ -52,8 +52,9 @@ abstract project Target_Options is
         --  link against libgnat (itself).
         --  Since -nostdlib also removes libgcc
         --  from the linked libraries we have to
-        --  add -lgcc again.
-        LOPTIONS := LOPTIONS & ("-nostdlib", "-lgcc");
+        --  add -lc and -lgcc again.
+        --  Also add -pthread in case it is not already included in -lc.
+        LOPTIONS := LOPTIONS & ("-nostdlib", "-lc", "-lgcc", "-pthread");
   end case;
 
   --  Concatenate with common flags