Android: don't force installation prefix

User should be able to change its value to any location.

See: https://cmake.org/cmake/help/v3.12/variable/CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT.html

CMakeLists.txt

@@ -123,7 +123,9 @@ if(SFML_OS_ANDROID)
     endif()
 
     # install everything in $NDK/sources/ because this path is appended by the NDK (convenient)
-    set(CMAKE_INSTALL_PREFIX ${CMAKE_ANDROID_NDK}/sources/third_party/sfml)
+    if(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
+        set(CMAKE_INSTALL_PREFIX ${CMAKE_ANDROID_NDK}/sources/third_party/sfml CACHE PATH "Installation path (should be inside your NDK's 'sources' directory)" FORCE)
+    endif()
 
     # we install libs in a subdirectory named after the ABI (lib/mips/*.so)
     set(LIB_SUFFIX "/${CMAKE_ANDROID_ARCH_ABI}")