Fix debug postfix in pkgconfig file

CMakeLists.txt uses set(CMAKE_DEBUG_POSTFIX "d") to distinguish
between debug and release lib. Use this postfix also in the
generated tinyxml2.pc file in case of a CMake Debug build.
This results in the following contents of tinyxml2.pc
- Release:
  Libs: -L${libdir} -ltinyxml2
- Debug:
  Libs: -L${libdir} -ltinyxml2d

CMakeLists.txt

@@ -106,6 +106,9 @@ endif()
 
 install(FILES tinyxml2.h DESTINATION ${CMAKE_INSTALL_INCLUDEDIR} COMPONENT tinyxml2_headers)
 
+if(CMAKE_BUILD_TYPE MATCHES Debug)
+  set(LIB_POSTFIX ${CMAKE_DEBUG_POSTFIX})
+endif()
 configure_file(tinyxml2.pc.in tinyxml2.pc @ONLY)
 install(FILES ${CMAKE_CURRENT_BINARY_DIR}/tinyxml2.pc DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig COMPONENT tinyxml2_config)