message: Add cmake option to choose where the forked SWIG source is fetched from
diff: @@ -2,6 +2,7 @@ option(STATIC_QRENDERDOC "Compile qrenderdoc as static" OFF)
  option(QRENDERDOC_NO_CXX11_REGEX "Disable C++11 regex in scintilla" OFF)
  set(QMAKE_QT5_COMMAND qmake CACHE STRING "Command to run to invoke Qt5's qmake. Normally this is qmake, possibly with qtchooser, but might be qmake-qt5")
  +set(RENDERDOC_SWIG_PACKAGE https://github.com/baldurk/swig/archive/renderdoc-3.0.12.zip CACHE STRING "The location where RenderDoc's swig fork source can be found. By default points to the URL on github but can be pointed to a local file.")
  set(QMAKE_CONFIG "debug")
  set(QMAKE_LDFLAGS "")
  @@ -67,7 +68,7 @@ find_package(BISON)
  # Compile our custom SWIG that will do scoped/strong enum classes
  ExternalProject_Add(custom_swig
  # using an URL to a zip directly so we don't clone the history etc
  - URL https://github.com/baldurk/swig/archive/renderdoc-3.0.12.zip
  + URL ${RENDERDOC_SWIG_PACKAGE}
  BUILD_IN_SOURCE 1
  CONFIGURE_COMMAND ./autogen.sh > /dev/null 2>&1
  COMMAND CC=${CMAKE_C_COMPILER} CXX=${CMAKE_CXX_COMPILER} ./configure --with-pcre=yes --prefix=${CMAKE_BINARY_DIR} > /dev/null
  