Changes to make xlisp.h usable in C++ code...

1) #include guard
2) "C" linkage for all function declarations
3) some uses of const

lib-src/libnyquist/nyquist/xlisp/xlftab.c

@@ -504,7 +504,7 @@ FUNDEF init_funtab[] = {
 FUNDEF *funtab = init_funtab;
 static size_t szfuntab = sizeof(init_funtab) / sizeof(*init_funtab);
 
-int xlbindfunctions(FUNDEF *functions, size_t nfunctions)
+int xlbindfunctions(const FUNDEF *functions, size_t nfunctions)
 {
    /* This is written very generally, imposing no fixed upper limit on the
     growth of the table.  But perhaps a lightweight alternative with such a

lib-src/libnyquist/nyquist/xlisp/xlisp.h

@@ -13,6 +13,13 @@ HISTORY
 
 /* system specific definitions */
 
+#ifndef __XLISP__
+#define __XLISP__
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #include <stdlib.h> /* needed for getenv(); note that this was a problem
     for PMAX implementation, but I assume PMAX is obsolete now. 
     - RBD 16apr04 */
@@ -366,7 +373,7 @@ void dbg_gc_xlsave(LVAL *n);
 
 /* function definition structure */
 typedef struct {
-    char *fd_name;	/* function name */
+    const char *fd_name;	/* function name */
     int fd_type;	/* function type */
     LVAL (*fd_subr)(void);	/* function entry point */
 } FUNDEF;
@@ -676,7 +683,7 @@ void xlsymbols(void);
    false if table limits would be exceeded and the table remains unchanged
    Call this, any number of times, before calling xlisp_main_init
  */
-int xlbindfunctions(FUNDEF *functions, size_t nfunctions);
+int xlbindfunctions(const FUNDEF *functions, size_t nfunctions);
 
 /* xlio.c */
 
@@ -1032,3 +1039,8 @@ void localinit(void);
 void localsymbols(void);
 void print_local_gc_info(void);
 
+#ifdef __cplusplus
+}
+#endif
+
+#endif