building.md: fix clang/clang++ arg

Whilst most versions of clang autodetect whether the code is C or C++ it seems
more recent versions do not do this correctly. Fix the doc to make sure this
works all the time

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
Signed-off-by: Sarah Knepper <sarah.knepper@intel.com>

docs/building.md

@@ -25,7 +25,7 @@ Building debug build:
  -DCMAKE_BUILD_TYPE=DEBUG
 
 Using clang instead of gcc:
- -DCMAKE_C_COMPILER=/usr/bin/clang -DCMAKE_CXX_COMPILER=/usr/bin/clang
+ -DCMAKE_C_COMPILER=/usr/bin/clang -DCMAKE_CXX_COMPILER=/usr/bin/clang++
 
 Building with an older version of swig (swig 2.0+) requires the disabling of javascript:
  -DBUILDSWIGNODE=OFF