cmake: Use GNUInstallDirs to install files to the correct paths

On most Linux distributions (excluding the Debian family), 64-bit
stuff goes into lib64, while 32-bit stuff goes into lib or lib32.
The Debian family has their own special setup as well.

This change makes it so we install correctly regardless of FHS
setup is used.

CMakeLists.txt

@@ -4,6 +4,8 @@ project(libdatachannel
 	LANGUAGES CXX)
 set(PROJECT_DESCRIPTION "C/C++ WebRTC network library featuring Data Channels, Media Transport, and WebSockets")
 
+include(GNUInstallDirs)
+
 # Options
 option(USE_MBEDTLS "Use Mbed TLS instead of OpenSSL" OFF)
 option(USE_GNUTLS "Use GnuTLS instead of OpenSSL" OFF)
@@ -445,13 +447,13 @@ if(WARNINGS_AS_ERRORS)
 endif()
 
 install(TARGETS datachannel EXPORT LibDataChannelTargets
-	RUNTIME DESTINATION bin
-	LIBRARY DESTINATION lib
-	ARCHIVE DESTINATION lib
+	RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
+	LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
+	ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
 )
 
 install(FILES ${LIBDATACHANNEL_HEADERS}
-	DESTINATION include/rtc
+	DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/rtc
 )
 
 # Export targets
@@ -459,13 +461,13 @@ install(
 	EXPORT LibDataChannelTargets
 	FILE LibDataChannelTargets.cmake
 	NAMESPACE LibDataChannel::
-	DESTINATION lib/cmake/LibDataChannel
+	DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/LibDataChannel
 )
 
 # Export config
 install(
 	FILES ${CMAKE_CURRENT_SOURCE_DIR}/cmake/LibDataChannelConfig.cmake
-	DESTINATION lib/cmake/LibDataChannel
+	DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/LibDataChannel
 )
 
 # Export config version
@@ -475,7 +477,7 @@ write_basic_package_version_file(
 	VERSION ${PROJECT_VERSION}
 	COMPATIBILITY SameMajorVersion)
 install(FILES ${CMAKE_BINARY_DIR}/LibDataChannelConfigVersion.cmake
-	DESTINATION lib/cmake/LibDataChannel)
+	DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/LibDataChannel)
 
 # Tests
 if(NOT NO_TESTS)