Miscellaneous travis-ci updates/fixes

- For the debian/buster docker build (GCC 7/C++17) install and use the
  system `catch` package; this also renames "COMPILER_PACKAGES" to
  "EXTRA_PACKAGES" since it now contains a non-compiler package.

- Add a status message indicating the catch version being used for
  compiling the embedded tests

- Simplify some bash code by using VAR+=" foo" to append (rather than
  VAR="${VAR} foo"

- Fix CMAKE_INCLUDE_PATH appending: it was prepending the ':' but not
  the existing $CMAKE_INCLUDE_PATH value and so would end up with
  ":/eigen-path" if CMAKE_INCLUDE_PATH was already set.  (This wasn't
  bug that was actually noticed since currently nothing else sets it).

tests/test_embed/CMakeLists.txt

@@ -5,7 +5,9 @@ if(${PYTHON_MODULE_EXTENSION} MATCHES "pypy")
 endif()
 
 find_package(Catch 1.9.3)
-if(NOT CATCH_FOUND)
+if(CATCH_FOUND)
+  message(STATUS "Building interpreter tests using Catch v${CATCH_VERSION}")
+else()
   message(STATUS "Catch not detected. Interpreter tests will be skipped. Install Catch headers"
                  " manually or use `cmake -DDOWNLOAD_CATCH=1` to fetch them automatically.")
   return()