(#17882) b2: Support CXX from a VirtualBuildEnv

* Use environment from VirtualBuildEnv to get CXX/CXXFLAGS

* Write a project-config.jam file containing the toolset from CXX

This ensures the b2 install command can find the correct compiler binary.

---------

Co-authored-by: Chris Mc <christopherm@jfrog.com>

recipes/b2/portable/conanfile.py

@@ -1,6 +1,7 @@
 from conan import ConanFile
 from conan.errors import ConanInvalidConfiguration
 from conan.tools.build import cross_building
+from conan.tools.env import VirtualBuildEnv
 from conan.tools.files import chdir, copy, get
 from conan.tools.layout import basic_layout
 
@@ -107,6 +108,12 @@ def _bootstrap_env(self):
             os.environ.clear()
             os.environ.update(saved_env)
 
+    def _write_project_config(self, cxx):
+        with open(os.path.join(self.source_folder, "project-config.jam"), "w") as f:
+            f.write(
+                f"using {self.options.toolset} : : {cxx} ;\n"
+            )
+
     def build(self):
         # The order of the with:with: below is important. The first one changes
         # the current dir. While the second does env changes that guarantees
@@ -139,10 +146,14 @@ def build(self):
         command += "build" if use_windows_commands else "./build.sh"
 
         if self.options.use_cxx_env:
-            cxx = os.environ.get("CXX")
+            envvars = VirtualBuildEnv(self).vars()
+
+            cxx = envvars.get("CXX")
             if cxx:
                 command += f" --cxx={cxx}"
-            cxxflags = os.environ.get("CXXFLAGS")
+                self._write_project_config(cxx)
+
+            cxxflags = envvars.get("CXXFLAGS")
             if cxxflags:
                 command += f" --cxxflags={cxxflags}"