Move interpreter shared library detection to configure

For building PL/Perl, PL/Python, and PL/Tcl, we need a shared library of
libperl, libpython, and libtcl, respectively.  Previously, this was
checked in the makefiles, skipping the PL build with a warning if no
shared library was available.  Now this is checked in configure, with an
error if no shared library is available.

The previous situation arose because in the olden days, the configure
options --with-perl, --with-python, and --with-tcl controlled whether
frontend interfaces for those languages would be built.  The procedural
languages were added later, and shared libraries were often not
available in the beginning.  So it was decided skip the builds of the
procedural languages in those cases.  The frontend interfaces have since
been removed from the tree, and shared libraries are now available most
of the time, so that setup makes much less sense now.

Also, the new setup allows contrib modules and pgxs users to rely on the
respective PLs being available based on configure flags.

configure

@@ -641,7 +641,6 @@ TCL_SHLIB_LD_LIBS
 TCL_SHARED_BUILD
 TCL_LIB_SPEC
 TCL_LIBS
-TCL_LIB_FILE
 TCL_INCLUDE_SPEC
 TCL_CONFIG_SH
 TCLSH
@@ -662,7 +661,6 @@ HAVE_IPV6
 LIBOBJS
 UUID_LIBS
 ZIC
-python_enable_shared
 python_additional_libs
 python_libspec
 python_libdir
@@ -7384,6 +7382,12 @@ perl_useshrplib=`$PERL -MConfig -e 'print $Config{useshrplib}'`
 test "$PORTNAME" = "win32" && perl_useshrplib=`echo $perl_useshrplib | sed 's,\\\\,/,g'`
 { $as_echo "$as_me:${as_lineno-$LINENO}: result: $perl_useshrplib" >&5
 $as_echo "$perl_useshrplib" >&6; }
+  if test "$perl_useshrplib" != yes && test "$perl_useshrplib" != true; then
+    as_fn_error $? "cannot build PL/Perl because libperl is not a shared library
+You might have to rebuild your Perl installation.  Refer to the
+documentation for details.  Use --without-perl to disable building
+PL/Perl." "$LINENO" 5
+  fi
 
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for flags to link embedded Perl" >&5
 $as_echo_n "checking for flags to link embedded Perl... " >&6; }
@@ -7537,6 +7541,32 @@ $as_echo "no" >&6; }
 fi
 
 
+
+  # We need libpython as a shared library.  With Python >=2.5, we check
+  # the Py_ENABLE_SHARED setting.  OS X does supply a .dylib even
+  # though Py_ENABLE_SHARED does not get set.  On Debian, the setting
+  # is not correct before the jessie release
+  # (http://bugs.debian.org/695979).  We also want to support older
+  # Python versions.  So as a fallback we see if there is a file that
+  # is named like a shared library.
+
+  if test "$python_enable_shared" != 1; then
+    # We don't know the platform shared library extension here yet, so
+    # we try some candidates.
+    for dlsuffix in .so .dll .dylib .sl; do
+      if ls "$python_libdir"/libpython*${dlsuffix}* >/dev/null 2>&1; then
+        python_enable_shared=1
+        break
+      fi
+    done
+  fi
+
+  if test "$python_enable_shared" != 1; then
+    as_fn_error $? "cannot build PL/Python because libpython is not a shared library
+You might have to rebuild your Python installation.  Refer to the
+documentation for details.  Use --without-python to disable building
+PL/Python." "$LINENO" 5
+  fi
 fi
 
 if test "$cross_compiling" = yes && test -z "$with_system_tzdata"; then
@@ -14736,12 +14766,15 @@ fi
 
     . "$TCL_CONFIG_SH"
 eval TCL_INCLUDE_SPEC=\"$TCL_INCLUDE_SPEC\"
-eval TCL_LIB_FILE=\"$TCL_LIB_FILE\"
 eval TCL_LIBS=\"$TCL_LIBS\"
 eval TCL_LIB_SPEC=\"$TCL_LIB_SPEC\"
 eval TCL_SHARED_BUILD=\"$TCL_SHARED_BUILD\"
 
-        # now that we have TCL_INCLUDE_SPEC, we can check for <tcl.h>
+        if test "$TCL_SHARED_BUILD" != 1; then
+      as_fn_error $? "cannot build PL/Tcl because Tcl is not a shared library
+Use --without-tcl to disable building PL/Tcl." "$LINENO" 5
+    fi
+    # now that we have TCL_INCLUDE_SPEC, we can check for <tcl.h>
     ac_save_CPPFLAGS=$CPPFLAGS
     CPPFLAGS="$TCL_INCLUDE_SPEC $CPPFLAGS"
     ac_fn_c_check_header_mongrel "$LINENO" "tcl.h" "ac_cv_header_tcl_h" "$ac_includes_default"