[build] use the default TARGETS installation folder when possible

It is implied by each target type (static and dynamic libraries here):

> For regular executables, static libraries and shared libraries, the
> DESTINATION argument is not required. For these target types, when
> DESTINATION is omitted, a default destination will be taken from the
> appropriate variable from GNUInstallDirs, or set to a built-in default value
> if that variable is not defined.

https://cmake.org/cmake/help/v3.14/command/install.html#targets

CMakeLists.txt

@@ -1074,12 +1074,24 @@ if (CYGWIN)
 endif()
 
 message(STATUS "INSTALL DIRS: bin=${CMAKE_INSTALL_BINDIR} lib=${CMAKE_INSTALL_LIBDIR} shlib=${INSTALL_SHARED_DIR} include=${CMAKE_INSTALL_INCLUDEDIR}")
-
-install(TARGETS ${INSTALL_TARGETS}
-		RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
-		ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
+if (NEED_DESTINATION)
+	if (DEFINED CMAKE_INSTALL_BINDIR AND DEFINED CMAKE_INSTALL_LIBDIR AND NOT INSTALL_SHARED_DIR STREQUAL "")
+		install(TARGETS ${INSTALL_TARGETS}
+			RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
+			ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
+			LIBRARY DESTINATION ${INSTALL_SHARED_DIR}
+		)
+	else()
+		message(WARNING "No location to install ${INSTALL_TARGETS}")
+	endif()
+elseif (NOT INSTALL_SHARED_DIR STREQUAL "")
+	install(TARGETS ${INSTALL_TARGETS}
 		LIBRARY DESTINATION ${INSTALL_SHARED_DIR}
-)
+	)
+else()
+	install(TARGETS ${INSTALL_TARGETS})
+endif()
+
 install(FILES ${HEADERS_srt} DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/srt)
 if (WIN32)
 	install(FILES ${HEADERS_srt_win32} DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/srt/win)