pbc: fix build

recipes/pbc/all/conanfile.py

@@ -3,6 +3,7 @@
 from conan import ConanFile
 from conan.tools.apple import XCRun, to_apple_arch
 from conan.tools.build import cross_building
+from conan.tools.env import VirtualBuildEnv
 from conan.tools.files import apply_conandata_patches, copy, export_conandata_patches, get, rm, rmdir, chdir
 from conan.tools.gnu import Autotools, AutotoolsToolchain, AutotoolsDeps
 from conan.tools.layout import basic_layout
@@ -66,8 +67,10 @@ def source(self):
         get(self, **self.conan_data["sources"][self.version], strip_root=True)
 
     def generate(self):
+        env = VirtualBuildEnv(self)
+        env.generate()
         tc = AutotoolsToolchain(self)
-        tc.extra_defines += ["LEX=flex"]
+        tc.configure_args.append("LEX=flex")
         # No idea why this is necessary, but if you don't set CC this way, then
         # configure complains that it can't find gmp.
         if cross_building(self) and self.settings.compiler == "apple-clang":