fix cmake option USE_POSTFIX (#283)

appending the suffix to the build product need not be a function of
whether CMAKE_CONFIGURATION_TYPES is set.  for example, having two ninja
build trees---one for debug and another for release---is a fine use-case
for USE_POSTFIX.

CMakeLists.txt

@@ -81,7 +81,7 @@ foreach(TARGET ${LIBRARY})
 		$<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}/src>
 		$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}${INSTALL_SUFFIX}>)
 
-	if(USE_POSTFIX AND CMAKE_CONFIGURATION_TYPES)
+	if(USE_POSTFIX)
 		set_target_properties(${TARGET} PROPERTIES DEBUG_POSTFIX "_d" MINSIZEREL_POSTFIX "_m" RELWITHDEBINFO_POSTFIX "_r")
 	endif()
 endforeach()