taskwarrior: Fix clang build

packages/taskwarrior/CMakeLists.txt.patch

@@ -0,0 +1,14 @@
+diff -u -r ../task-2.5.1/CMakeLists.txt ./CMakeLists.txt
+--- ../task-2.5.1/CMakeLists.txt	2016-02-24 23:18:11.000000000 +0100
++++ ./CMakeLists.txt	2017-01-15 23:14:01.677031016 +0100
+@@ -43,10 +43,6 @@
+  message (FATAL_ERROR "C++11 support missing. Try upgrading your C++ compiler. If you have a good reason for using an outdated compiler, please let us know at support@taskwarrior.org.")
+ endif (_HAS_CXX11)
+ 
+-if (${CMAKE_CXX_COMPILER_ID} STREQUAL "Clang")
+-  set (_CXX11_FLAGS "${_CXX11_FLAGS} -stdlib=libc++")
+-endif (${CMAKE_CXX_COMPILER_ID} STREQUAL "Clang")
+-
+ if (${CMAKE_SYSTEM_NAME} MATCHES "Linux")
+   set (LINUX true)
+ elseif (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")