Skip test for IPC::Run if user is overriding our search for PROVE.

The check for IPC::Run we added in commit c254970a is useful in simple
cases, but there are real use-cases where "prove" is coming from a
different Perl installation than the "perl" we want to use to build.
In such cases asking whether "perl" knows about IPC::Run is irrelevant
and can cause an unnecessary configure failure.  Hence, if user has
specified a value for PROVE, skip the IPC::Run check.  Per discussion
with Andrew Dunstan.

Discussion: https://postgr.es/m/E1dcE5n-0005Sk-UE@gemulon.postgresql.org

configure

@@ -16457,67 +16457,11 @@ fi
 # Check for test tools
 #
 if test "$enable_tap_tests" = yes; then
+  # Check for necessary modules, unless user has specified the "prove" to use;
+  # in that case it's her responsibility to have a working configuration.
+  # (prove might be part of a different Perl installation than perl, eg on
+  # MSys, so the result of AX_PROG_PERL_MODULES could be irrelevant anyway.)
   if test -z "$PROVE"; then
-  for ac_prog in prove
-do
-  # Extract the first word of "$ac_prog", so it can be a program name with args.
-set dummy $ac_prog; ac_word=$2
-{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
-$as_echo_n "checking for $ac_word... " >&6; }
-if ${ac_cv_path_PROVE+:} false; then :
-  $as_echo_n "(cached) " >&6
-else
-  case $PROVE in
-  [\\/]* | ?:[\\/]*)
-  ac_cv_path_PROVE="$PROVE" # Let the user override the test with a path.
-  ;;
-  *)
-  as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
-for as_dir in $PATH
-do
-  IFS=$as_save_IFS
-  test -z "$as_dir" && as_dir=.
-    for ac_exec_ext in '' $ac_executable_extensions; do
-  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
-    ac_cv_path_PROVE="$as_dir/$ac_word$ac_exec_ext"
-    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
-    break 2
-  fi
-done
-  done
-IFS=$as_save_IFS
-
-  ;;
-esac
-fi
-PROVE=$ac_cv_path_PROVE
-if test -n "$PROVE"; then
-  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $PROVE" >&5
-$as_echo "$PROVE" >&6; }
-else
-  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
-$as_echo "no" >&6; }
-fi
-
-
-  test -n "$PROVE" && break
-done
-
-else
-  # Report the value of PROVE in configure's output in all cases.
-  { $as_echo "$as_me:${as_lineno-$LINENO}: checking for PROVE" >&5
-$as_echo_n "checking for PROVE... " >&6; }
-  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $PROVE" >&5
-$as_echo "$PROVE" >&6; }
-fi
-
-  if test -z "$PROVE"; then
-    as_fn_error $? "prove not found" "$LINENO" 5
-  fi
-  if test -z "$PERL"; then
-    as_fn_error $? "Perl not found" "$LINENO" 5
-  fi
-  # Check for necessary modules
 
 
 
@@ -16595,6 +16539,65 @@ else
   { $as_echo "$as_me:${as_lineno-$LINENO}: WARNING: could not find perl" >&5
 $as_echo "$as_me: WARNING: could not find perl" >&2;}
 fi
+  fi
+  # Now make sure we know where prove is
+  if test -z "$PROVE"; then
+  for ac_prog in prove
+do
+  # Extract the first word of "$ac_prog", so it can be a program name with args.
+set dummy $ac_prog; ac_word=$2
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
+$as_echo_n "checking for $ac_word... " >&6; }
+if ${ac_cv_path_PROVE+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  case $PROVE in
+  [\\/]* | ?:[\\/]*)
+  ac_cv_path_PROVE="$PROVE" # Let the user override the test with a path.
+  ;;
+  *)
+  as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in $PATH
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_exec_ext in '' $ac_executable_extensions; do
+  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
+    ac_cv_path_PROVE="$as_dir/$ac_word$ac_exec_ext"
+    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
+    break 2
+  fi
+done
+  done
+IFS=$as_save_IFS
+
+  ;;
+esac
+fi
+PROVE=$ac_cv_path_PROVE
+if test -n "$PROVE"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $PROVE" >&5
+$as_echo "$PROVE" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+
+
+  test -n "$PROVE" && break
+done
+
+else
+  # Report the value of PROVE in configure's output in all cases.
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking for PROVE" >&5
+$as_echo_n "checking for PROVE... " >&6; }
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $PROVE" >&5
+$as_echo "$PROVE" >&6; }
+fi
+
+  if test -z "$PROVE"; then
+    as_fn_error $? "prove not found" "$LINENO" 5
+  fi
 fi
 
 # Thread testing

configure.in

@@ -2134,16 +2134,19 @@ PGAC_PATH_PROGS(FOP, fop)
 # Check for test tools
 #
 if test "$enable_tap_tests" = yes; then
+  # Check for necessary modules, unless user has specified the "prove" to use;
+  # in that case it's her responsibility to have a working configuration.
+  # (prove might be part of a different Perl installation than perl, eg on
+  # MSys, so the result of AX_PROG_PERL_MODULES could be irrelevant anyway.)
+  if test -z "$PROVE"; then
+    AX_PROG_PERL_MODULES(IPC::Run, ,
+      AC_MSG_ERROR([Perl module IPC::Run is required to run TAP tests]))
+  fi
+  # Now make sure we know where prove is
   PGAC_PATH_PROGS(PROVE, prove)
   if test -z "$PROVE"; then
     AC_MSG_ERROR([prove not found])
   fi
-  if test -z "$PERL"; then
-    AC_MSG_ERROR([Perl not found])
-  fi
-  # Check for necessary modules
-  AX_PROG_PERL_MODULES(IPC::Run, ,
-    AC_MSG_ERROR([Perl module IPC::Run is required to run TAP tests]))
 fi
 
 # Thread testing