Demonstrate C and C++ flag propagation (#549)

Drake now listens to the CMAKE_CXX_FLAGS and CMAKE_C_FLAGS cmake variables.
To demonstrate this behavior, these flags are now forwarded to drake as part
of the drake_cmake_external example.

Author: tom-osika

drake_cmake_external/CMakeLists.txt

@@ -119,6 +119,8 @@ ExternalProject_Add(drake
   CMAKE_ARGS
     -DCMAKE_BUILD_TYPE:STRING=${CMAKE_BUILD_TYPE}
     -DCMAKE_C_COMPILER:FILEPATH=${CMAKE_C_COMPILER}
+    -DCMAKE_C_FLAGS:STRING=${CMAKE_C_FLAGS}
+    -DCMAKE_CXX_FLAGS:STRING=${CMAKE_CXX_FLAGS}
     -DCMAKE_INSTALL_PREFIX:PATH=${CMAKE_INSTALL_PREFIX}
     -DCMAKE_PREFIX_PATH:PATH=${CMAKE_PREFIX_PATH}
     -DPYTHON_EXECUTABLE:FILEPATH=${Python3_EXECUTABLE}
@@ -147,6 +149,7 @@ ExternalProject_Add(drake_external_examples
   CMAKE_ARGS
     -DCMAKE_BUILD_TYPE:STRING=${CMAKE_BUILD_TYPE}
     -DCMAKE_CXX_COMPILER:FILEPATH=${CMAKE_CXX_COMPILER}
+    -DCMAKE_C_FLAGS:STRING=${CMAKE_C_FLAGS}
     -DCMAKE_CXX_FLAGS:STRING=${CMAKE_CXX_FLAGS}
     -DCMAKE_INSTALL_PREFIX:PATH=${CMAKE_INSTALL_PREFIX}
     -DCMAKE_PREFIX_PATH:PATH=${CMAKE_PREFIX_PATH}