maint: use STREQ/STRNEQ.

* doc/bison.texinfo: Space change.
	* src/system.h (STREQ, STRNEQ): New.
	* src/files.c, src/ielr.c, src/lalr.c, src/muscle-tab.c,
	* src/output.c, src/print.c, src/print_graph.c,
	* src/reader.c, src/scan-skel.l, src/tables.c,
	* src/uniqstr.c:
	Use them.
	* src/scan-gram.l: Do not use streq.h, use system.h's STREQ.
	* cfg.mk: The documentation is an exception.

src/system.h

@@ -39,6 +39,9 @@
 #include <stdlib.h>
 #include <string.h>
 
+#define STREQ(L, R)  (strcmp(L, R) == 0)
+#define STRNEQ(L, R) (!STREQ(L, R))
+
 #if HAVE_SYS_TYPES_H
 # include <sys/types.h>
 #endif