Reapply 6b2a219e26

Author: Paul Licameli <paul.licameli@audacityteam.org>
    Date:   Tue Feb 27 13:51:10 2018 -0500

        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

@@ -424,7 +424,7 @@ 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     for PMAX implementation, but I assume PMAX is obsolete now.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 */
@@ -378,7 +385,7 @@ 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;
@@ -688,7 +695,7 @@ 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 */
 
@@ -1055,3 +1062,10 @@ void localinit(void);
 void localsymbols(void);
 void print_local_gc_info(void);
 void local_toplevel(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
+