tools/bison: hardcode path to m4 with STAGING_DIR_HOST

Force bison to ignore the M4 environment variable and hardcode it to the
locally built m4 during build operations using the relocatable path
variable STAGING_DIR_HOST.

This allows bison to continue to function while we are forcefully avoiding
autoreconf and other autoconf and automake-like operations by giving a
fake path to m4 with the M4 environment variable.

The specific path can still be overridden independently from the
environment within the line of invocation that runs bison by setting
STAGING_DIR_HOST within the command, so document this in the help printout.

Tested-by: Georgi Valkov <[email protected]> # MacOS
Signed-off-by: Michael Pratt <[email protected]>
Signed-off-by: Tony Ambardar <[email protected]>

tools/bison/patches/000-relocatable.patch

@@ -0,0 +1,43 @@
+--- a/src/files.c
++++ b/src/files.c
+@@ -560,9 +560,9 @@ pkgdatadir (void)
+ char const *
+ m4path (void)
+ {
+-  char const *m4 = getenv ("M4");
++  char const *m4 = getenv ("STAGING_DIR_HOST");
+   if (m4)
+-    return m4;
++    return strcat(getenv ("STAGING_DIR_HOST"), "/bin/m4");
+ 
+   /* We don't use relocate2() to store the temporary buffer and re-use
+      it, because m4path() is only called once.  */
+--- a/src/getargs.c
++++ b/src/getargs.c
+@@ -373,11 +373,13 @@ usage (int status)
+          A --long option is required.
+          Otherwise, add exceptions to ../build-aux/cross-options.pl.  */
+ 
+-      printf (_("Usage: %s [OPTION]... FILE\n"), program_name);
++      printf (_("Usage: STAGING_DIR_HOST=... %s [OPTION]... FILE\n"), program_name);
+       fputs (_("\
+ Generate a deterministic LR or generalized LR (GLR) parser employing\n\
+ LALR(1), IELR(1), or canonical LR(1) parser tables.\n\
+ \n\
++Environment Variable STAGING_DIR_HOST controls path to m4\n\
++\n\
+ "), stdout);
+ 
+       fputs (_("\
+@@ -450,6 +452,11 @@ Output Files:\n\
+   -M, --file-prefix-map=OLD=NEW replace prefix OLD with NEW when writing file paths\n\
+                                 in output files\n\
+ "), stdout);
++
++      fputs (_("\
++Environment Variables:\n\
++  STAGING_DIR_HOST              Path to m4 is [STAGING_DIR_HOST]/bin/m4\n\
++"), stdout);
+       putc ('\n', stdout);
+ 
+       argmatch_report_usage (stdout);