[build] Fix Build for Android. (#2100)

Some versions of CMake set CMAKE_SYSTEM_NAME to "Android".
Link the atomic library for Android targets.

CMakeLists.txt

@@ -39,7 +39,7 @@ set_if(LINUX       ${CMAKE_SYSTEM_NAME} MATCHES "Linux")
 set_if(BSD         ${SYSNAME_LC} MATCHES "bsd$")
 set_if(MICROSOFT   WIN32 AND (NOT MINGW AND NOT CYGWIN))
 set_if(GNU         ${CMAKE_SYSTEM_NAME} MATCHES "GNU")
-set_if(ANDROID     ${CMAKE_SYSTEM_NAME} MATCHES "ANDROID")
+set_if(ANDROID     "${CMAKE_SYSTEM_NAME}" MATCHES "^[Aa][Nn][Dd][Rr][Oo][Ii][Dd]")
 set_if(POSIX       LINUX OR DARWIN OR BSD OR ANDROID OR (CYGWIN AND CYGWIN_USE_POSIX))
 set_if(SYMLINKABLE LINUX OR DARWIN OR BSD OR CYGWIN OR GNU)
 
@@ -867,6 +867,8 @@ if (srt_libspec_shared)
 		target_link_libraries(${TARGET_srt}_shared PRIVATE wsock32.lib ws2_32.lib)
 	elseif (APPLE)
 		set_property(TARGET ${TARGET_srt}_shared PROPERTY MACOSX_RPATH ON)
+	elseif (ANDROID)
+		target_link_libraries(${TARGET_srt}_shared PRIVATE atomic)
 	endif()
 	if (USE_GNUSTL)
 		target_link_libraries(${TARGET_srt}_shared PRIVATE ${GNUSTL_LIBRARIES} ${GNUSTL_LDFLAGS})
@@ -901,6 +903,8 @@ if (srt_libspec_static)
 		endif()
 	elseif (MINGW)
 		target_link_libraries(${TARGET_srt}_static PRIVATE wsock32 ws2_32)
+	elseif (ANDROID)
+		target_link_libraries(${TARGET_srt}_static PUBLIC atomic)
 	endif()
 	if (USE_GNUSTL)
 		target_link_libraries(${TARGET_srt}_static PRIVATE ${GNUSTL_LIBRARIES} ${GNUSTL_LDFLAGS})