Don't use PKG_INSTALLDIR just yet, it breaks pkg-config < 0.27.

This reverts commit 7c714a8.

Makefile

@@ -9,7 +9,7 @@ include buildsys.mk
 install-extra:
 	for i in audacious.pc audclient.pc; do \
 	        ${INSTALL_STATUS}; \
-		if ${MKDIR_P} ${DESTDIR}${pkgconfigdir} && ${INSTALL} -m 644 $$i ${DESTDIR}${pkgconfigdir}/$$i; then \
+		if ${MKDIR_P} ${DESTDIR}${libdir}/pkgconfig && ${INSTALL} -m 644 $$i ${DESTDIR}${libdir}/pkgconfig/$$i; then \
 			${INSTALL_OK}; \
 		else \
 			${INSTALL_FAILED}; \

buildsys.mk.in

@@ -73,7 +73,6 @@ prefix = @prefix@
 exec_prefix = @exec_prefix@
 bindir = @bindir@
 libdir = @libdir@
-pkgconfigdir = @pkgconfigdir@
 plugindir ?= ${libdir}/${PACKAGE_NAME}
 datarootdir = @datarootdir@
 datadir = @datadir@

configure.ac

@@ -23,8 +23,6 @@ BUILDSYS_INIT
 BUILDSYS_SHARED_LIB
 AC_DEFINE_UNQUOTED(PLUGIN_SUFFIX, "$PLUGIN_SUFFIX", [Suffix for plugins])
 
-PKG_INSTALLDIR
-
 dnl Byte order
 dnl ==========
 AC_C_BIGENDIAN([BIGENDIAN=1], [BIGENDIAN=0],