configure: simplify shell viability test.

* configure: the golden trick is simply to test
whether PS4 contains "+ ". PS2 is not a useful variable,
because even crappy old 1980's vintage System V shells
set that up. It turns out that Zsh in POSIX mode (when run as
/bin/sh) handles configuring and building TXR just fine.
And Zsh does set PS4 to "+ " in that mode. However, it
does not set PS2, unless run interactively. When run
interactively as /bin/sh, Zsh sets PS2 to "> ".
So, let us drop the Bash variable tests, and the test
for PS2 being "> " and only test PS4.

1 files changed, 6 insertions, 14 deletions

diff --git a/configure b/configure
index 75e1604e..180a3966 100755
--- a/configure
+++ b/configure
@@ -39,20 +39,12 @@ while true ; do
     break
   fi
 
-  # Evidence we are running on Bash in POSIX mode.
-  if test x$POSIXLY_CORRECT = y ; then
-    break
-  fi
-
-  # Evidence we are running on Bash in regular mode.
-  if test x$BASH_VERSION != x ; then
-    break
-  fi
-
-  # If the shell has PS2 and PS4 with these values, it is
-  # probably a good shell: late-model Ash, Dash or the XPG
-  # shell on Solaris and perhaps others.
-  if test "x$PS2" = "x> " && test "x$PS4" = "x+ " ; then
+  # If PS4 is set to "+ ", we are probably running on a good shell: GNU
+  # Bash sets it like this, as does late-model Ash, Dash, Korn Shell 93,
+  # and the XPG shell on Solaris 10. Zsh sets PS4 to "+ " this in its
+  # POSIX mode, which handles our script, but to some other value in its
+  # regular mode which doesn't handle our script.
+  if test "x$PS4" = "x+ " ; then
     break
   fi