build: account for msys2's duplicity in path schemes

wrapper.py.in

@@ -76,12 +76,41 @@ defaults = {
 env = defaults.copy()
 env.update(os.environ)
 
+# MinGW pretends to be its own platform. Try to work around their deception by
+# testing if the platform string looks like something that's actually useful.
+def is_really_windows() -> bool:
+    name = platform.system()
+
+    if name == 'Windows':
+        return True
+    if name.startswith('MINGW'):
+        return True
+    return False
+
+
+# MSYS2 wants to fuck us over by supplying a CMake that gives something
+# approximating native paths, and by _also_ providing an environment that
+# cannot use these paths in places like PATH.
+#
+# Break the paths to conform to their expectations if we're under Windows.
+def break_path_for_msys2(path):
+    if not is_really_windows():
+        return path
+
+    # Test if we have a path of the form "C:/foo" (as opposed to a
+    # relative path, or an MSYS2 path like "/c/foo")
+    if path[1] != ':':
+        return path
+
+    return '/' + path[0] + path[2:]
+
+
 # Windows has its own unique rules for library lookups. We record the
 # environment variable which effects path lookups and the separator and the
 # target directory for Windows and for every single other system we're likely
 # to ever deal with...
-if platform.system() == 'Windows':
+if is_really_windows():
-    separator = ';'
+    separator = ':'
     depsdir = ['bin']
     searchvar = 'PATH'
 else:
@@ -92,7 +121,7 @@ else:
 
 # append the in-tree dependencies to the library path
 search = separator.join(
-    "@CMAKE_CURRENT_BINARY_DIR@/deps/" + i for i in depsdir
+    break_path_for_msys2("@CMAKE_CURRENT_BINARY_DIR@/deps/") + i for i in depsdir
 )
 
 if searchvar in env: