Use TEST_CPPFLAGS/TEST_CXXFLAGS to add path to CxxTest headers in configure.ac. ...

Use TEST_CPPFLAGS/TEST_CXXFLAGS to add path to CxxTest headers in configure.ac. (#200)

CxxTest headers were included in test/unit/Makefile.am as -I@CXXTEST@. However, in configure.ac if CXXTEST was not set by the user, it was initially set to `dirname "$CXXTESTGEN"` while determining the path to the header files. If CXXTESTGEN pointed to /usr/bin and if the headers were found in /usr/include, CXXTEST was not reset, which led to CXXTEST being replaced by /usr/bin in test/unit/Makefile.am. As a consequence, the locale binary in /usr/bin got included instead of the locale header file.