Update configure script to include option to enable and disable PDF man page
generation.

--- a/configure.ac
+++ b/configure.ac
@@ -27,10 +27,16 @@
 AC_CONFIG_HEADER([config.h])
 
 AC_DEFINE([_GNU_SOURCE], [], [Enable various GNU extensions])
 AC_DEFINE([_STDC_FORMAT_MACROS], [], [printf-style format macros])
 
+AC_ARG_ENABLE([pdf-doc],
+	      AS_HELP_STRING([--disable-pdf-doc], [Disable PDF documentation]),
+	      AS_IF([test "x$enable_pdf_doc" = "xno"], [enable_pdf_doc=no],
+	      [enable_pdf_doc=yes]), [enable_pdf_doc=yes])
+AM_CONDITIONAL([BUILD_PDF], [test "x$enable_pdf_doc" == "xyes" ])
+
 AC_ARG_ENABLE([debug],
 	      AS_HELP_STRING([--enable-debug], [Disable debugging]),
 	      AS_IF([test "x$enable_debug" = "xno"], [with_debug=no], [with_debug=yes]),
 	      [with_debug=yes])
 AC_SUBST(with_debug)
@@ -61,15 +67,15 @@
 	)]
 )
 AC_SUBST(DB2MAN)
 AM_CONDITIONAL([BUILD_MAN], [test -n "$DB2MAN"])
 
-AC_CHECK_PROG(DBLATEX, [dblatex], [found], [no])
-AS_IF([test "$DBLATEX" == "no"],
-	[AC_MSG_WARN([dblatex not found, no PDF manpages will be built])]
-)
-AM_CONDITIONAL([BUILD_PDF], [test "$DBLATEX" == "found"])
+AM_COND_IF([BUILD_PDF], [
+	AC_CHECK_PROG(DBLATEX, [dblatex], [found], [no])
+	AS_IF([test "$DBLATEX" == "no"],
+	      [AC_MSG_ERROR([dblatex not found])])
+])
 
 # Checks for libraries.
 PKG_CHECK_MODULES([LIBMNL], [libmnl >= 1.0.3])
 PKG_CHECK_MODULES([LIBNFTNL], [libnftnl >= 1.0.5])
 
@@ -134,6 +140,7 @@
 
 echo "
 nft configuration:
   cli support:			${with_cli}
   enable debugging:		${with_debug}
-  use mini-gmp:			${with_mini_gmp}"
+  use mini-gmp:			${with_mini_gmp}
+  enable pdf documentation:	${enable_pdf_doc}"