cmake: use -std=c++11 instead of c++14

With GCC 9, some of the stdlibc++'s header files don't compile with
AspectC++ anymore.  Until this is resolved, we compile with -std=c++11
although we actually use C++14 features; we need to ignore the
warnings pointing this out until AspectC++ works with these header
files.

Change-Id: If4d3c9fcbee61b7f8c6ffdf7f1ef9a641b6b6743

cmake/compilerconfig.cmake

@@ -1,7 +1,7 @@
 #### C++14 ####
 # We need at least C++11, as some library headers begin to require it.  C++14
 # has already aged sufficiently to mandate it here.
-set(CMAKE_CXX_STANDARD 14)
+set(CMAKE_CXX_STANDARD 11)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)