Exclude unresolved symbol scheme_signals_registered on Win32 builds

Commit 72d90e4 ("library-manager, numeric, and bytevector-compres
improvements") introduced a regression causing Win32 nmake builds to
fail due to an undefined symbol in c\schsig.c.

This symbol (scheme_signals_registered) is defined in a preprocessor-
conditional code block excluding WIN32 between lines 544 and 737.

This commit bypasses the build regression by enclosing the unresolved
expression in a preprocessor conditional excluding WIN32.

See GitHub Issue #497 for more details:
https://github.com/cisco/ChezScheme/issues/497

original commit: 10bccf39badee76f80d87326d2fc7c4d808fa08e

c/schsig.c

@@ -770,7 +770,9 @@ void S_schsig_init() {
 
         S_protect(&S_G.error_id);
         S_G.error_id = S_intern((const unsigned char *)"$c-error");
+#ifndef WIN32
         scheme_signals_registered = 0;
+#endif
     }