Detect gcc stdlib for gcc 4.0.1.

For some reason the normal macros aren't defined for the standard
library that comes with gcc 4.0.1 (although maybe just on BSDs?). So try
to detect the library for that compiler.

[SVN r52245]

test/Jamfile.v2

@@ -10,6 +10,8 @@ project hash-tests
         <toolset>gcc:<define>_GLIBCXX_DEBUG
         <toolset>gcc:<cxxflags>-Wsign-promo
         #<toolset>gcc:<cxxflags>-Wextra
+        <toolset>darwin:<define>_GLIBCXX_DEBUG
+        <toolset>darwin:<cxxflags>-Wsign-promo
     ;
 
 test-suite functional/hash