message: Add --extra-flags= option to configure script
diff: @@ -25,6 +25,7 @@ Usage: $0 [OPTION]... [VAR=VALUE]...
  - MinSizeRel: minimal output size
  - Release: optimizations on, debugging off
  - RelWithDebInfo: release flags plus debugging
  + --extra-flags=STRING additional compiler flags
  --build-dir=DIR place build files in directory [build]
  --bin-dir=DIR executable directory [build/bin]
  --lib-dir=DIR library directory [build/lib]
  @@ -87,7 +88,7 @@ Usage: $0 [OPTION]... [VAR=VALUE]...
  Influential Environment Variables (only on first invocation):
  CXX C++ compiler command
  - CXXFLAGS C++ compiler flags
  + CXXFLAGS C++ compiler flags (overrides defaults)
  LDFLAGS Additional linker flags
  CMAKE_GENERATOR Selects a custom generator
  @@ -319,6 +320,9 @@ while [ $# -ne 0 ]; do
  --build-type=*)
  append_cache_entry CMAKE_BUILD_TYPE STRING "$optarg"
  ;;
  + --extra-flags=*)
  + append_cache_entry EXTRA_FLAGS STRING "$optarg"
  + ;;
  --build-dir=*)
  builddir="$optarg"
  ;;
  