[Milhouse]

CXXFLAGS += -std=c++11 should help
if memory serves g++ 5.3 in ubuntu 16.04 defaults to std=gnu++98, which lacks lots of stuff,
however this should be checked and set properly by cmake, if it's not than it's a bug in the package itself and should be reported to package maintainers.

Yeah... from 16.04 VM (which fails):

text:
-- The C compiler identification is GNU 5.4.0

-- The CXX compiler identification is GNU 5.4.0

-- Check for working C compiler: /home/ubuntu/projects/LibreELEC.tv/build.LibreELEC-RPi.arm-9.0-devel/toolchain/bin/host-gcc

-- Check for working C compiler: /home/ubuntu/projects/LibreELEC.tv/build.LibreELEC-RPi.arm-9.0-devel/toolchain/bin/host-gcc -- works

-- Detecting C compiler ABI info

-- Detecting C compiler ABI info - done

-- Detecting C compile features

-- Detecting C compile features - done

-- Check for working CXX compiler: /home/ubuntu/projects/LibreELEC.tv/build.LibreELEC-RPi.arm-9.0-devel/toolchain/bin/host-g++

-- Check for working CXX compiler: /home/ubuntu/projects/LibreELEC.tv/build.LibreELEC-RPi.arm-9.0-devel/toolchain/bin/host-g++ -- works

and from 17.10 (which works):

text:
-- The C compiler identification is GNU 7.2.0

-- The CXX compiler identification is GNU 7.2.0

-- Check for working C compiler: /home/neil/projects/scratch/alternates/LibreELEC.tv/build.LibreELEC-Generic.x86_64-9.0-devel-gcc/toolchain/bin/host-gcc

-- Check for working C compiler: /home/neil/projects/scratch/alternates/LibreELEC.tv/build.LibreELEC-Generic.x86_64-9.0-devel-gcc/toolchain/bin/host-gcc -- works

-- Detecting C compiler ABI info

-- Detecting C compiler ABI info - done

-- Detecting C compile features

-- Detecting C compile features - done

-- Check for working CXX compiler: /home/neil/projects/scratch/alternates/LibreELEC.tv/build.LibreELEC-Generic.x86_64-9.0-devel-gcc/toolchain/bin/host-g++

-- Check for working CXX compiler: /home/neil/projects/scratch/alternates/LibreELEC.tv/build.LibreELEC-Generic.x86_64-9.0-devel-gcc/toolchain/bin/host-g++ -- works

Both are wrong, of course, as in both cases it's building with gcc-8.1.0 - what the package is doing is detecting the build host gcc version not the $TOOLCHAIN version (even though that is what it claims to be using). So it's possibly a little more involved than setting CXXFLAGS.