make pkgdatadir relative to program path if not specified

[Victor Zverovich]

Currently running bison using a path other than the install path e.g.
via a network share results in the following error

bison: .../share/bison/m4sugar/m4sugar.m4: cannot open: No such file
or directory

Fix this by setting setting pkgdatadir relative to program path if
BISON_PKGDATADIR is not specified.
---
 src/output.c | 32 ++++++++++++++++++++++++++++++--
 1 file changed, 30 insertions(+), 2 deletions(-)

diff --git a/src/output.c b/src/output.c
index 96295d17..749aad6d 100644
--- a/src/output.c
+++ b/src/output.c
@@ -21,14 +21,17 @@
 #include <config.h>
 #include "system.h"

+#include <errno.h>
 #include <configmake.h>
 #include <filename.h> /* IS_PATH_WITH_DIR */
 #include <get-errno.h>
+#include <libgen.h>
 #include <path-join.h>
 #include <quotearg.h>
 #include <spawn-pipe.h>
 #include <timevar.h>
 #include <wait-process.h>
+#include <progname.h>

 #include "complain.h"
 #include "files.h"
@@ -701,6 +704,31 @@ output (void)
 char const *
 pkgdatadir (void)
 {
-  char const *cp = getenv ("BISON_PKGDATADIR");
-  return cp ? cp : PKGDATADIR;
+  static char default_pkgdatadir[PATH_MAX] = "";
+
+  char const *pkgdatadir = getenv ("BISON_PKGDATADIR");
+  if (pkgdatadir) {
+    return pkgdatadir;
+  }
+
+  if (default_pkgdatadir[0] == '\0') {
+    static const char *rel_pkgdatadir = "/../share/bison";
+    char path[PATH_MAX];
+
+    if (realpath (program_name, path) == NULL) {
+      fprintf (stderr, "unable to resolve realpath of \"%s\": %s",
+        program_name, strerror (errno));
+      abort ();
+    }
+
+    dirname (path);
+    strncat (path, rel_pkgdatadir, PATH_MAX - 1 - strlen (rel_pkgdatadir));
+    if (realpath (path, default_pkgdatadir) == NULL) {
+      fprintf (stderr, "unable to resolve realpath of \"%s\": %s",
+        path, strerror (errno));
+      abort ();
+    }
+  }
+
+  return default_pkgdatadir;
 }
--
2.13.5