Escape CMake config var in configure.ac

Previously ${CMAKE_CURRENT_LIST_DIR} was expanded (to an empty string) before CapnProtoConfig.cmake.in was configured, resulting in CapnProtoConfig.cmake setting CAPNP_INCLUDE_DIRECTORY to "/../../include", which is obviously wrong.

c++/configure.ac

@@ -126,7 +126,7 @@ AC_SUBST([CAPNP_CMAKE_CONFIG_FILES])
 
 # CapnProtoConfig.cmake.in needs these PACKAGE_* output variables.
 PACKAGE_INIT="set([CAPNP_PKG_CONFIG_FILES] CAPNP_PKG_CONFIG_FILES)"
-PACKAGE_CMAKE_INSTALL_FULL_INCLUDEDIR="${CMAKE_CURRENT_LIST_DIR}/../../include"
+PACKAGE_CMAKE_INSTALL_FULL_INCLUDEDIR="\${CMAKE_CURRENT_LIST_DIR}/../../../include"
 AC_SUBST([PACKAGE_INIT])
 AC_SUBST([PACKAGE_CMAKE_INSTALL_FULL_INCLUDEDIR])