misc. fixes

- tccgen: error out for cast to void, as in
      void foo(void) { return 1; }
  This avoids an assertion failure in x86_64-gen.c, also.
  also fix tests2/03_struct.c accordingly

- Error: "memory full" - be more specific

- Makefiles: remove circular dependencies, lookup tcctest.c from VPATH

- tcc.h: cleanup lib, include, crt and libgcc search paths"
  avoid duplication or trailing slashes with no CONFIG_MULTIARCHDIR
  (as from 9382d6f1a0)

- tcc.h: remove ";{B}" from PE search path
  in ce5e12c2f9 James Lyon wrote:
  "... I'm not sure this is the right way to fix this problem."
  And the answer is: No, please. (copying libtcc1.a for tests instead)

- win32/build_tcc.bat: do not move away a versioned file

libtcc.c

@@ -214,7 +214,7 @@ PUB_FUNC void *tcc_malloc(unsigned long size)
     void *ptr;
     ptr = malloc(size);
     if (!ptr && size)
-        tcc_error("memory full");
+        tcc_error("memory full (malloc)");
 #ifdef MEM_DEBUG
     mem_cur_size += malloc_usable_size(ptr);
     if (mem_cur_size > mem_max_size)
@@ -239,7 +239,7 @@ PUB_FUNC void *tcc_realloc(void *ptr, unsigned long size)
 #endif
     ptr1 = realloc(ptr, size);
     if (!ptr1 && size)
-        tcc_error("memory full");
+        tcc_error("memory full (realloc)");
 #ifdef MEM_DEBUG
     /* NOTE: count not correct if alloc error, but not critical */
     mem_cur_size += malloc_usable_size(ptr1);