Produce a clean failure in configure when dtrace is selected but cannot be

found.  Besides stopping those early who have no dtrace installed
whatsoever, this will also alert those who have dtrace in /usr/sbin, which
might not be in the path, which would produce confusing failures much later
in the build process.

Add documentation about pointing configure to find dtrace.

configure

@@ -2004,6 +2004,11 @@ fi
   test -n "$DTRACE" && break
 done
 
+if test -z "$DTRACE"; then
+  { { echo "$as_me:$LINENO: error: dtrace not found" >&5
+echo "$as_me: error: dtrace not found" >&2;}
+   { (exit 1); exit 1; }; }
+fi
 
       ;;
     no)

configure.in

 dnl Process this file with autoconf to produce a configure script.
-dnl $PostgreSQL: pgsql/configure.in,v 1.471 2006/08/04 15:16:14 tgl Exp $
+dnl $PostgreSQL: pgsql/configure.in,v 1.472 2006/08/17 17:25:43 petere Exp $
 dnl
 dnl Developers, please strive to achieve this order:
 dnl
@@ -213,6 +213,9 @@ PGAC_ARG_BOOL(enable, dtrace, no,
 [AC_DEFINE([ENABLE_DTRACE], 1, 
            [Define to 1 to enable DTrace support. (--enable-dtrace)])
 AC_CHECK_PROGS(DTRACE, dtrace)
+if test -z "$DTRACE"; then
+  AC_MSG_ERROR([dtrace not found])
+fi
 AC_SUBST(DTRACEFLAGS)])
 AC_SUBST(enable_dtrace)
 

doc/src/sgml/installation.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/installation.sgml,v 1.259 2006/07/24 16:32:44 petere Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/installation.sgml,v 1.260 2006/08/17 17:25:43 petere Exp $ -->
 
 <chapter id="installation">
  <title><![%standalone-include[<productname>PostgreSQL</>]]>
@@ -1011,10 +1011,24 @@ su - postgres
        <term><option>--enable-dtrace</option></term>
        <listitem>
         <para>
+         <indexterm>
+          <primary>DTrace</primary>
+         </indexterm>
          Compiles with support for the dynamic tracing tool DTrace.
          Operating system support for DTrace is currently only
          available in Solaris.
         </para>
+
+        <para>
+         To point to the <command>dtrace</command> program, the
+         environment variable <envar>DTRACE</envar> can be set.  This
+         will often be necessary because <command>dtrace</command> is
+         typically installed under <filename>/usr/sbin</filename>,
+         which might not be in the path.  Additional command-line
+         options for the <command>dtrace</command> program can be
+         specified in the environment variable
+         <envar>DTRACEFLAGS</envar>.
+        </para>
        </listitem>
       </varlistentry>
 
@@ -1113,6 +1127,25 @@ su - postgres
        </listitem>
       </varlistentry>
 
+      <varlistentry>
+       <term><option>DTRACE=<replaceable>/path/to/dtrace</></option></term>
+       <listitem>
+        <para>
+         Specifies the location of the <command>dtrace</command> program.
+        </para>
+       </listitem>
+      </varlistentry>
+
+      <varlistentry>
+       <term><option>DTRACEFLAGS=<replaceable>FLAGS</></option></term>
+       <listitem>
+        <para>
+         <replaceable>FLAGS</> is the a list of flags to pass to the
+         <command>dtrace</command> program.
+        </para>
+       </listitem>
+      </varlistentry>
+
       <varlistentry>
        <term><option>JADE</option></term>
        <listitem>