build system (Perl bindings): Make Makefile.PL's PREFIX/INSTALL_BASE …

…optional.

Both options may cause problems:

 - PREFIX might clash with INSTALL_BASE being set in $PERL_MM_OPT.
 - INSTALL_BASE without any further options set will install into non-standard
   directories in many cases.

Rather, use PREFIX=${prefix} as default but overwrite that if
--with-perl-bindings=... has been used.

Thanks to faxm0dem for reporting the INSTALL_BASE problems on IRC and
bvarner and nirik for reporting this in GH collectd#177 and collectd#196.

This is a follow-up / fix for 97e8f34.

bindings/Makefile.am

@@ -48,7 +48,7 @@ perl: buildperl/Makefile
 
 buildperl/Makefile: .perl-directory-stamp buildperl/Makefile.PL \
 	$(top_builddir)/config.status
-	cd buildperl && @PERL@ Makefile.PL INSTALL_BASE=$(prefix) @PERL_BINDINGS_OPTIONS@
+	cd buildperl && @PERL@ Makefile.PL @PERL_BINDINGS_OPTIONS@
 
 buildperl/Makefile.PL: .perl-directory-stamp $(top_builddir)/config.status
 

configure.in

@@ -5062,19 +5062,18 @@ then
 fi
 
 dnl Perl bindings
+PERL_BINDINGS_OPTIONS="PREFIX=${prefix}"
 AC_ARG_WITH(perl-bindings, [AS_HELP_STRING([--with-perl-bindings@<:@=OPTIONS@:>@], [Options passed to "perl Makefile.PL".])],
 [
 	if test "x$withval" != "xno" && test "x$withval" != "xyes"
 	then
 		PERL_BINDINGS_OPTIONS="$withval"
 		with_perl_bindings="yes"
 	else
-		PERL_BINDINGS_OPTIONS=""
 		with_perl_bindings="$withval"
 	fi
 ],
 [
-	PERL_BINDINGS_OPTIONS=""
 	if test -n "$perl_interpreter"
 	then
 		with_perl_bindings="yes"