build: Properly escape $( inside perl regex

Use one back slash instead of two, otherwise perl was replacing it
with the $( (aka $GID).

Broken due to commit 02f1f8a50af58b9d1f61dd9258cf1d26fe2f0d21.

m4/perl.m4

@@ -5,5 +5,5 @@ AC_DEFUN([DPKG_PROG_PERL],
 [AC_ARG_VAR([PERL], [Perl interpreter])dnl
 AC_PATH_PROG([PERL], [perl], [/usr/bin/perl])dnl
 AC_ARG_VAR([PERL_LIBDIR], [Perl library directory])dnl
-PERL_LIBDIR=$($PERL -MConfig -e 'my $r=$Config{vendorlibexp};$r=~s/$Config{vendorprefixexp}/\\$(prefix)/; print $r')dnl
+PERL_LIBDIR=$($PERL -MConfig -e 'my $r=$Config{vendorlibexp};$r=~s/$Config{vendorprefixexp}/\$(prefix)/; print $r')dnl
 ])# DPKG_PROG_PERL