C files should be compiled with equivalent flags as the C++ files

Impala's be/CMakeLists.txt does calculations to determine CMAKE_CXX_FLAGS, but it does not set CMAKE_C_FLAGS. This means that C files don't get the options that we specify on C++ files. This is pretty minor, because there are very few C files in the codebase. Even so, it would be good to specify similar flags for CMAKE_C_FLAGS when we set CMAKE_CXX_FLAGS.

As an example, be/src/thirdparty/squeasel/squeasel.c won't be compiled with '-fno-omit-frame-pointer' or debug information for release builds. If it fails during webserver startup (e.g. when it tries to read the private key to setup SSL), the resulting stack in the minidump is jumbled a bit.

This should be relatively easy to fix, but it may involve reorganizing the way we calculate CMAKE_CXX_FLAGS.