setup: find system python path for CI hack

The problem appears to be that virtualenv overrides the base_prefix
so distutils can no longer find the system python path when adding
linker args.

This adds an ugly hack to find the system python path, and adds it
to the linker flags on Windows. Yay, no more hardcoded paths.

This should also make win32 builds work, and avoid adding linker
args manually.

setup.py

@@ -18,7 +18,7 @@ from distutils.command.build_ext import build_ext
 from distutils.command.sdist import sdist
 from distutils.cmd import Command
 from distutils.dir_util import remove_tree
-from distutils.sysconfig import get_python_inc, get_python_lib
+from distutils.sysconfig import get_python_inc
 from distutils import log
 import os, os.path
 import glob
@@ -103,6 +103,17 @@ def recursive_package_data(src, package_dir='overviewer_core'):
 
     return ret
 
+# Finds the system-wide path from within a venv.
+# Taken from https://github.com/pyinstaller/pyinstaller/blob/master/PyInstaller/hooks/pre_find_module_path/hook-distutils.py
+def find_system_module_path():
+    # opcode is not a virtualenv module, so we can use it to find the stdlib. Technique taken from virtualenv's
+    # "distutils" package detection at
+    # https://github.com/pypa/virtualenv/blob/16.3.0/virtualenv_embedded/distutils-init.py#L5
+    import opcode
+
+    system_module_path = os.path.normpath(os.path.dirname(opcode.__file__))
+    return system_module_path
+
 #
 # py2exe options
 #
@@ -183,10 +194,20 @@ c_overviewer_includes = ['overviewer.h', 'rendermodes.h']
 c_overviewer_files = ['overviewer_core/src/' + s for s in c_overviewer_files]
 c_overviewer_includes = ['overviewer_core/src/' + s for s in c_overviewer_includes]
 
+# really ugly hack for our scuffed CI, remove this once we move
+# to something else. The problem is that virtualenv somehow
+# now overrides the base_prefix (which it shouldn't do) which
+# makes distutils unable to find our Python library
+python_lib_dir = ""
+if platform.system() == 'Windows':
+    ci_python_dir = os.path.split(find_system_module_path())[0]
+    python_lib_dir = os.path.join(ci_python_dir, "Libs")
+
 setup_kwargs['ext_modules'].append(Extension(
     'overviewer_core.c_overviewer',
     c_overviewer_files,
     include_dirs=['.', numpy_include] + pil_include,
+    library_dirs=[python_lib_dir],
     depends=c_overviewer_includes,
     extra_link_args=[]
 ))
@@ -296,9 +317,6 @@ class CustomBuildExt(build_ext):
                 e.extra_link_args.append("/MANIFEST")
                 e.extra_link_args.append("/DWINVER=0x060")
                 e.extra_link_args.append("/D_WIN32_WINNT=0x060")
-                # workaround for our extremely shitty build system
-                # yeet this into a fire once we move to github actions
-                e.extra_link_args.append("/LIBPATH:C:\Python37\Libs")
         if c == "unix":
             # customize the build options for this compilier
             for e in self.extensions: