update mingw toolchain file while we're at it

still does not compile, it'll require its own set of changes to make
the code gcc-compliant

cmake/toolchains/mingw.cmake

@@ -1,11 +1,42 @@
+# TODO: make this work
+# Usage:
+# cmake --toolchain cmake/toolchains/mingw.cmake ...
+#
+# Unlike clang-cl.cmake this needs no MSVC_SDK: mingw-w64 ships its own Windows
+# headers and import libraries in its sysroot, so the compiler finds everything
+# itself. Install the i686 cross toolchain (mingw-w64-gcc) and it is on PATH as
+# i686-w64-mingw32-*.
+
 set(CMAKE_SYSTEM_NAME Windows)
 set(CMAKE_SYSTEM_PROCESSOR i686)
 
 set(triple ${CMAKE_SYSTEM_PROCESSOR}-w64-mingw32)
 
-set(CMAKE_C_COMPILER ${triple}-gcc)
-set(CMAKE_CXX_COMPILER ${triple}-g++)
-set(CMAKE_RC_COMPILER ${triple}-windres)
+# Find every tool here rather than trusting a bare name, so the toolchain file
+# alone decides which cross compiler, rc and dlltool are used.
+foreach(tool gcc g++ windres dlltool ar)
+  string(TOUPPER "${tool}" _variable)
+  string(REPLACE "+" "X" _variable "${_variable}")
+  find_program(MINGW_${_variable} NAMES ${triple}-${tool} REQUIRED)
+endforeach()
+
+set(CMAKE_C_COMPILER "${MINGW_GCC}")
+set(CMAKE_CXX_COMPILER "${MINGW_GXX}")
+set(CMAKE_RC_COMPILER "${MINGW_WINDRES}")
+set(CMAKE_AR "${MINGW_AR}")
 
 set(CMAKE_C_COMPILER_TARGET ${triple})
 set(CMAKE_CXX_COMPILER_TARGET ${triple})
+
+# GNU ld cannot bind the Bink import library the game ships either, so rebuild a
+# bindable one from binkw32.def (which explains why) and hand it to the top-level
+# CMakeLists through binkw32_lib. MSVC keeps using the shipped library.
+execute_process(
+  COMMAND "${MINGW_DLLTOOL}" -m i386
+          -d "${CMAKE_CURRENT_LIST_DIR}/../../binkw32.def"
+          -l "${CMAKE_BINARY_DIR}/binkw32.lib"
+  RESULT_VARIABLE _binkw32DlltoolResult)
+if(NOT _binkw32DlltoolResult EQUAL 0)
+  message(FATAL_ERROR "dlltool failed to build the binkw32 import library")
+endif()
+set(binkw32_lib "${CMAKE_BINARY_DIR}/binkw32.lib")