swift-lldb compilation fails with c++11 error - swift

Question

I am trying to compile swift-lldb on Ubuntu 14.04 (following instructions from https://github.com/apple/swift-lldb). I have the following dependencies installed:
Clang-3.5
Cmake version 3.5.2
Python version 2.7.6
On running the build script step which is lldb/scripts/build-swift-cmake.py --test, I am seeing the following error:
CMake Warning at cmake/modules/HandleLLVMOptions.cmake:185 (message):
-fPIC is not supported.
Call Stack (most recent call first):
cmake/modules/HandleLLVMOptions.cmake:216 (add_flag_or_print_warning)
CMakeLists.txt:616 (include)
CMake Error at cmake/modules/HandleLLVMOptions.cmake:429 (message):
LLVM requires C++11 support but the '-std=c++11' flag isn't supported.
Call Stack (most recent call first):
CMakeLists.txt:616 (include)
I have defined environment variables CC and CXX to point to the clang C and C++ compilers.
root:/myswift# echo $CC
/usr/bin/clang
root:/myswift# echo $CXX
/usr/bin/clang++
I also found in the clang documentation that c++11 is supported by clang-3.5. Not sure what I am missing here. Can someone please help?

Answer 1

clang-4.0 mentioned as part of the installation should support the -std=c++11 flag (just tested clang-4.0.1). However, upgrading to clang-6.0 seems to solve this build process error.
Running cmake directly in the automatically created build directory (by the swift build scripts) could be used to investigate the build failure in more detail. To specify compilers here, rather than setting CC and CXX environment variables (which works well for GNU configure scripts), compilers can be set for cmake via
cmake -DCMAKE_C_COMPILER=/usr/bin/clang -DCMAKE_CXX_COMPILER=/usr/bin/clang++ path_to_src_or_build_directory
An existing CMakeCache.txt might have to be removed, so that the above parameters are honored.
A problem with clang++ installations that I have observed is that clang++ cannot find C++ headers (i.e. if C++ headers are in non-standard locations other than /usr/include, unlikely in the case of Ubuntu though). In case /usr/bin/clang++ cannot compile a simple program like
#include <iostream>
using namespace std;
int main() {
cout << "hello" << endl;
return 0;
}
not being able to find the iostream include file, it might help to set --gcc-toolchain=/pathtoaworkinggcc, where pathtoaworkinggcc should include include, lib, bin, etc. of a working C++ compiler (possibly g++ in the case of a Ubuntu installation).