configure: Fix installation of library to work with alternate --prefix

If an explicit --libdir is passed, then that is used directly. Otherwise
libdir is chosen as the value of $PREFIX/lib, (whether or not prefix was
passed explicitly or set by default).

configure

@@ -11,7 +11,7 @@ XAPIAN_CONFIG=${XAPIAN_CONFIG:-xapian-config-1.1 xapian-config}
 # Set the defaults for values the user can specify with command-line
 # options.
 PREFIX=/usr/local
-LIBDIR=${PREFIX}/lib
+LIBDIR=
 
 usage ()
 {
@@ -50,7 +50,6 @@ Additionally, various options can be specified on the configure
 command line.
 
 	--prefix=PREFIX	Install files in PREFIX [$PREFIX]
-	--libdir=LIBDIR	Install libraries in LIBDIR [$LIBDIR]
 
 By default, "make install" will install the resulting program to
 $PREFIX/bin, documentation to $PREFIX/man, etc. You can
@@ -59,6 +58,10 @@ specify an installation prefix other than $PREFIX using
 
 	./configure --prefix=\$HOME
 
+Fine tuning of some installation directories is available:
+
+	--libdir=DIR	Install libraries in LIBDIR [PREFIX/lib]
+
 EOF
 }
 
@@ -276,7 +279,7 @@ CXXFLAGS = ${CXXFLAGS}
 prefix = ${PREFIX}
 
 # The directory to which notmuch libraries should be installed
-libdir = ${LIBDIR}
+libdir = ${LIBDIR:=$PREFIX/lib}
 
 # The directory to which emacs lisp files should be installed
 emacs_lispdir=${emacs_lispdir}