Add testing of C++ with sanitizers (CI-Docker) (#5631)

* Add C++ build testing with clang and gcc

This adds Dockerfiles which test building flatc and the C++ library against clang
and gcc. See discussion at #5119.  It is derived from the Travis CI tooling.

The GRPC tests are failing due to #5099 so those are commented out.

These are run from the .travis.yml file rather than the tests/docker/languages
folder because the builds may each take longer than 30 minutes and were hitting
Travis timeouts.

Parallel builds and build caching attempt to keep the build times low.

* Add GCC 8.3 and Clang 7.0 with sanitizers into CI (based on #5130)

- Add a docker based on Debian Buster.
- Add C++ building scripts for the docker.
- Leak-sanitizer requires SYS_PTRACE.

CMakeLists.txt

@@ -13,6 +13,8 @@ option(FLATBUFFERS_BUILD_FLATLIB "Enable the build of the flatbuffers library"
        ON)
 option(FLATBUFFERS_BUILD_FLATC "Enable the build of the flatbuffers compiler"
        ON)
+option(FLATBUFFERS_STATIC_FLATC "Build flatbuffers compiler with -static flag"
+       OFF)
 option(FLATBUFFERS_BUILD_FLATHASH "Enable the build of flathash" ON)
 option(FLATBUFFERS_BUILD_GRPCTEST "Enable the build of grpctest" OFF)
 option(FLATBUFFERS_BUILD_SHAREDLIB
@@ -295,6 +297,9 @@ if(FLATBUFFERS_BUILD_FLATC)
     # Make flatc.exe not depend on runtime dlls for easy distribution.
     target_compile_options(flatc PUBLIC $<$<CONFIG:Release>:/MT>)
   endif()
+  if(FLATBUFFERS_STATIC_FLATC AND NOT MSVC)
+    target_link_libraries(flatc PRIVATE -static)
+  endif()
 endif()
 
 if(FLATBUFFERS_BUILD_FLATHASH)