Android: the native app glue needs C language enabled.

This was not caught by Travis CI, because there an older CMake version
is used and in that version I'm enabling C because otherwise nothing
works at all.

Sorry.

CMakeLists.txt

@@ -44,8 +44,9 @@ if(POLICY CMP0054)
 endif()
 
 # Configuration fails on < 3.4 if only C++ is enabled (CheckFunctionExists
-# macro called from FindX11)
-if(NOT CMAKE_VERSION VERSION_LESS 3.4.0)
+# macro called from FindX11). Also, Android needs C for compiling some glue
+# code.
+if(NOT CMAKE_VERSION VERSION_LESS 3.4.0 AND NOT CMAKE_SYSTEM_NAME STREQUAL "Android")
     set(LANG CXX)
 endif()
 project(Magnum ${LANG})