Allow pkgconfigdir to be set when running configure

This also makes autogen.sh compatible with older versions of autotools +
pkg-config.

Makefile.am

@@ -54,6 +54,7 @@ nobase_dist_graphtooldoc_DATA = \
 
 EXTRA_DIST = $(graphtooldoc_DATA) autogen.sh
 
+pkgconfigdir = @pkgconfigdir@
 pkgconfig_DATA = graph-tool-py${PYTHON_VERSION}.pc
 
 # Copy all the spec files. Of cource, only one is actually used.

configure.ac

@@ -31,8 +31,8 @@ AC_LANG([C++])
 AC_PROG_CC_STDC
 AC_HEADER_STDC
 AC_DISABLE_STATIC
-LT_INIT
 AC_PROG_INSTALL
+LT_INIT
 
 # pkg-config generation
 PKG_PROG_PKG_CONFIG
@@ -382,6 +382,16 @@ else
 fi
 AC_SUBST(PACKAGE_DOC_DIR)
 
+# Allow the pkg-config directory to be set
+AC_ARG_WITH(pkgconfigdir,
+            AC_HELP_STRING([--with-pkgconfigdir],
+            [Use the specified pkgconfig dir (default is libdir/pkgconfig)]),
+            [pkgconfigdir=${withval}],
+            [pkgconfigdir='${libdir}/pkgconfig'])
+
+AC_SUBST([pkgconfigdir])
+AC_MSG_NOTICE([pkgconfig directory is ${pkgconfigdir}])
+
 dnl Subst PYTHON_DIR.
 AC_DEFINE_UNQUOTED([INSTALL_PREFIX],"${prefix}", [python prefix])
 AC_DEFINE_UNQUOTED([PYTHON_DIR], "${PYTHON_SITE_PKG}", [The directory name for the site-packages subdirectory of the standard Python install tree.])