Use CXXFLAGS when compiling parsers (#197)

Use CXXFLAGS when compiling parsers (#197)

ANTLR generates C files that we compile with the C++ compiler.
To do so, we set CC=CXX in the `Makefile.am`s of the parsers.
Previously, we did not copy the CXXFLAGS to the CFLAGS, which
could result in problems, e.g. when using -std=gnu++11 in the
CXXFLAGS, compiling the parsers would fail if they used C++11
features (configure.ac usually modifies CXX to include the
-std=gnu++11 flag but if it is included in CXXFLAGS, the CXX
is not changed).