libtcc: minor adjustments

- use {B} to substitute tcc_lih_path (instead of \b)

- expand CONFIG_TCC_CRTPREFIX in CONFIG_TCC_LIBPATHS
  which fixes duplicate CONFIG_SYSROOT.

- put default CONFIG_SYSROOT ("") into tcc.h

- remove hack from commit db6fcce78f
  because $(tccdir)/include is already in sysincludes

- configure: error out for unrecognized options.

- win32/build-tcc.bat: put libtcc into base dir where it will
  find lib/include automatically, and build libtcc_test example.

libtcc.h

@@ -1,10 +1,8 @@
 #ifndef LIBTCC_H
 #define LIBTCC_H
 
-#ifdef LIBTCC_AS_DLL
-#define LIBTCCAPI __declspec(dllexport)
-#else
-#define LIBTCCAPI
+#ifndef LIBTCCAPI
+# define LIBTCCAPI
 #endif
 
 #ifdef __cplusplus