Update libgit2 version from 0.20.0 to 0.21.1

build_tools/XCompile-nacl.cmake

 # Cross-compiling requires CMake 2.6 or newer. To cross-compile, first modify
 # this file to set the proper settings and paths. Then use it from build/ like:
 # cmake .. -DCMAKE_TOOLCHAIN_FILE=../XCompile.txt \
 #          -DCMAKE_INSTALL_PREFIX=/usr/mingw32/mingw
 # If you already have a toolchain file setup, you may use that instead of this
 # file.
 
+SET(NACL 1)
+
 # the name of the target operating system
 SET(CMAKE_SYSTEM_NAME Linux)
 
 # which compilers to use for C and C++
 SET(CMAKE_C_COMPILER "${NACLCC}")
 SET(CMAKE_CXX_COMPILER "${NACLCXX}")
 SET(CMAKE_LINKER "${NACLLD}")
 SET(CMAKE_AR "${NACLAR}" CACHE FILEPATH "Archiver")
 SET(_CMAKE_TOOLCHAIN_PREFIX ${NACL_CROSS_PREFIX})
 
 # here is the target environment located
 SET(CMAKE_FIND_ROOT_PATH ${NACL_TOOLCHAIN_ROOT})
 
 # adjust the default behaviour of the FIND_XXX() commands:
 # search headers and libraries in the target environment, search 
 # programs in the host environment
 SET(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
 SET(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
 SET(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
 SET(CMAKE_REQUIRED_INCLUDES "${NACL_SDK_ROOT}/include")
 INCLUDE_DIRECTORIES(${NACL_SDK_ROOT}/include)
 INCLUDE_DIRECTORIES(${EXTRA_INCLUDE})
 
 # nacl abi says 32bits little endian
 SET(CMAKE_SIZEOF_VOID_P 4)