Fix a C++ standard conflict between pocl & LLVM

for files compiled with LLVM flags, always let the LLVM flags
determine the C++ standard (-std=XYZ) instead of using pocl's
project-wide setting (CMAKE_CXX_STANDARD)

lib/CL/CMakeLists.txt

@@ -23,6 +23,11 @@
 #
 #=============================================================================
 
+# this is required, because we're compiling the C++ files of this directory
+# with LLVM-provided flags, which usually contain their own setting of
+# C++ standard, which may be in conflict with pocl's default C++ standard.
+unset(CMAKE_CXX_STANDARD)
+
 if(SANITIZER_OPTIONS)
   add_compile_options(${SANITIZER_OPTIONS})
 endif()

lib/llvmopencl/CMakeLists.txt

@@ -23,6 +23,11 @@
 #
 #=============================================================================
 
+# this is required, because we're compiling these files with LLVM-provided flags, which
+# usually contain their own setting of C++ standard, which may be in conflict
+# with pocl's default C++ standard.
+unset(CMAKE_CXX_STANDARD)
+
 set(LLVMPASSES_SOURCES "AllocasToEntry.cc"
                        "AllocasToEntry.h"
                        "AutomaticLocals.cc"