Replace CL_REQUIRE/CL_PROVIDE(cl_st_null) with portable code.

The order of initialization of non-local objects in different compilation units
is not specified in C++. Hence special care should be taken to avoid static
initialization order fiasco. CLN solved the problem with some evil (GCC
specific, and even GCC-version-specific) hack. Replace it with a technique
similar to one used in STL to initialize std::cout and friends.

include/cln/string.h

@@ -124,10 +124,9 @@ inline cl_string::operator cl_heap_string* () const
 }
 inline cl_string::cl_string ()
 {
-	extern const cl_string cl_null_string;
-	pointer = (cl_heap_string*) cl_null_string;
+	static const cl_string cl_null_st(NULL, 0);
+	pointer = (cl_heap_string*) cl_null_st;
 }
-CL_REQUIRE(cl_st_null)
 
 // Hash code.
 extern unsigned long hashcode (const cl_string& str);

src/base/string/cl_st_null.cc

@@ -3,13 +3,12 @@
 // General includes.
 #include "cl_sysdep.h"
 
-CL_PROVIDE(cl_st_null)
-
 // Specification.
 #include "cln/string.h"
 
 namespace cln {
 
+#warning "Please remove me"
 extern const cl_string cl_null_string;
 
 }  // namespace cln
@@ -22,4 +21,3 @@ const cl_string cl_null_string = cl_string(NULL,0);
 
 }  // namespace cln
 
-CL_PROVIDE_END(cl_st_null)