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

win32/build-tcc.bat

@@ -63,5 +63,5 @@ del *.o
 echo>..\config.texi @set VERSION %VERSION%
 if not exist doc md doc
 makeinfo --html --no-split -o doc\tcc-doc.html ../tcc-doc.texi
-if exist tcc-win32.txt move tcc-win32.txt doc\
-copy ..\tests\libtcc_test.c examples\
+copy tcc-win32.txt doc
+copy ..\tests\libtcc_test.c examples