See for details:
http://lists.altlinux.org/pipermail/devel/2009-March/167877.html

diff --git a/fluxbox/configure.in b/fluxbox/configure.in
index 916f4c8..7b3c6c7 100644
--- a/fluxbox/configure.in
+++ b/fluxbox/configure.in
@@ -444,8 +444,8 @@ if test x$enable_imlib2 = "xyes"; then
       [
          IMLIB2=true
          AC_DEFINE(HAVE_IMLIB2, [], [Imlib2 support])
-         IMLIB2_LIBS=`imlib2-config --libs`
-         IMLIB2_CFLAGS=`imlib2-config --cflags`
+         IMLIB2_LIBS=`pkg-config imlib2 --libs`
+         IMLIB2_CFLAGS=`pkg-config imlib2 --cflags`
          LIBS="$LIBS $IMLIB2_LIBS"
          CXXFLAGS="$CXXFLAGS $IMLIB2_CFLAGS"
        ], [ AC_MSG_RESULT(no)]