Add LLVM c++_static c++_shared STL targets

The Android NDK would be moving to Clang as its default compiler
in the future. This patch allows OpenCV to build with the default
c++_shared / c++_static

TEST=Builds OpenCV with -DANDROID_TOOLCHAIN_NAME=
aarch64-linux-android-clang3.6 -DANDROID_STL=c++_shared

platforms/android/android.toolchain.cmake

@@ -155,6 +155,10 @@
 #                          Implies -frtti -fno-exceptions.
 #                          Available for NDK r7b and newer.
 #                          Silently degrades to gnustl_static if not available.
+#        c++_static     -> Use the LLVM libc++ runtime as a static library.
+#                          Implies -frtti -fexceptions.
+#        c++_shared     -> Use the LLVM libc++ runtime as a static library.
+#                          Implies -frtti -fno-exceptions.
 #
 #    ANDROID_STL_FORCE_FEATURES=ON - turn rtti and exceptions support based on
 #      chosen runtime. If disabled, then the user is responsible for settings
@@ -842,7 +846,7 @@ set( ANDROID_STL_FORCE_FEATURES ON CACHE BOOL "automatically configure rtti and
 mark_as_advanced( ANDROID_STL ANDROID_STL_FORCE_FEATURES )
 
 if( BUILD_WITH_ANDROID_NDK )
- if( NOT "${ANDROID_STL}" MATCHES "^(none|system|system_re|gabi\\+\\+_static|gabi\\+\\+_shared|stlport_static|stlport_shared|gnustl_static|gnustl_shared)$")
+ if( NOT "${ANDROID_STL}" MATCHES "^(none|system|system_re|gabi\\+\\+_static|gabi\\+\\+_shared|stlport_static|stlport_shared|gnustl_static|gnustl_shared|c\\+\\+_static|c\\+\\+_shared)$")
   message( FATAL_ERROR "ANDROID_STL is set to invalid value \"${ANDROID_STL}\".
 The possible values are:
   none           -> Do not configure the runtime.
@@ -854,15 +858,19 @@ The possible values are:
   stlport_shared -> Use the STLport runtime as a shared library.
   gnustl_static  -> (default) Use the GNU STL as a static library.
   gnustl_shared  -> Use the GNU STL as a shared library.
+  c++_shared     -> Use the LLVM libc++ runtime as a shared library.
+  c++_static     -> Use the LLVM libc++ runtime as a static library.
 " )
  endif()
 elseif( BUILD_WITH_STANDALONE_TOOLCHAIN )
- if( NOT "${ANDROID_STL}" MATCHES "^(none|gnustl_static|gnustl_shared)$")
+ if( NOT "${ANDROID_STL}" MATCHES "^(none|gnustl_static|gnustl_shared|c\\+\\+_static|c\\+\\+_shared)$")
   message( FATAL_ERROR "ANDROID_STL is set to invalid value \"${ANDROID_STL}\".
 The possible values are:
   none           -> Do not configure the runtime.
   gnustl_static  -> (default) Use the GNU STL as a static library.
   gnustl_shared  -> Use the GNU STL as a shared library.
+  c++_shared     -> Use the LLVM libc++ runtime as a shared library.
+  c++_static     -> Use the LLVM libc++ runtime as a static library.
 " )
  endif()
 endif()
@@ -1035,9 +1043,17 @@ if( BUILD_WITH_ANDROID_NDK )
   else()
    set( __libstl                "${__libstl}/libs/${ANDROID_NDK_ABI_NAME}/libstdc++.a" )
   endif()
+ elseif( ANDROID_STL MATCHES "c\\+\\+" )
+  set( ANDROID_EXCEPTIONS       ON )
+  set( ANDROID_RTTI             ON )
+  set( __libstl                "${ANDROID_NDK}/sources/cxx-stl/llvm-libc++" )
+  set( __libstl                "${__libstl}/libs/${ANDROID_NDK_ABI_NAME}/libc++_static.a" )
+  set( __libgnustl             "${ANDROID_NDK}/sources/cxx-stl/gnu-libstdc++/${ANDROID_COMPILER_VERSION}" )
+  set( ANDROID_STL_INCLUDE_DIRS "${__libgnustl}/include" "${__libgnustl}/libs/${ANDROID_NDK_ABI_NAME}/include" "${__libgnustl}/include/backward" )
  else()
   message( FATAL_ERROR "Unknown runtime: ${ANDROID_STL}" )
  endif()
+
  # find libsupc++.a - rtti & exceptions
  if( ANDROID_STL STREQUAL "system_re" OR ANDROID_STL MATCHES "gnustl" )
   set( __libsupcxx "${ANDROID_NDK}/sources/cxx-stl/gnu-libstdc++/${ANDROID_COMPILER_VERSION}/libs/${ANDROID_NDK_ABI_NAME}/libsupc++.a" ) # r8b or newer
@@ -1067,7 +1083,9 @@ endif()
 # case of shared STL linkage
 if( ANDROID_STL MATCHES "shared" AND DEFINED __libstl )
  string( REPLACE "_static.a" "_shared.so" __libstl "${__libstl}" )
- # TODO: check if .so file exists before the renaming
+ if( NOT EXISTS "${__libstl}" )
+   message( FATAL_ERROR "Unable to find shared library ${__libstl}" )
+ endif()
 endif()