Use a test for PDF support that is more portable across teTeX major versions.

Backport of Python's revision 39232. Needed on cygwin.
diff --git a/doc/tools/texinputs/manual.cls b/doc/tools/texinputs/manual.cls
index 789cae1..942cb49 100644
--- a/doc/tools/texinputs/manual.cls
+++ b/doc/tools/texinputs/manual.cls
@@ -6,6 +6,7 @@
 \ProvidesClass{manual}
              [1998/03/03 Document class (Python manual)]
 
+\RequirePackage{ifpdf}
 \RequirePackage{pypaper}
 
 % Change the options here to get a different set of basic options, but only
@@ -23,7 +24,7 @@
 % distribution.
 %
 % The "fancyhdr" package makes nicer page footers reasonable to
-% implement, and is used to put the chapter and section information in 
+% implement, and is used to put the chapter and section information in
 % the footers.
 %
 \RequirePackage{fancyhdr}\typeout{Using fancier footers than usual.}
@@ -63,7 +64,8 @@
     \let\footnotesize\small
     \let\footnoterule\relax
     \py@doHorizontalRule%
-    \@ifundefined{pdfinfo}{}{{
+    \ifpdf
+      \begingroup
       % This \def is required to deal with multi-line authors; it
       % changes \\ to ', ' (comma-space), making it pass muster for
       % generating document info in the PDF file.
@@ -72,7 +74,8 @@
         /Author (\@author)
         /Title (\@title)
       }
-    }}
+      \endgroup
+    \fi
     \begin{flushright}%
       {\rm\Huge\py@HeaderFamily \@title \par}%
       {\em\LARGE\py@HeaderFamily \py@release \par}