chore: initialize build2 project

build/.gitignore

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+/config.build
+/root/
+/bootstrap/
+build/

build/bootstrap.build

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+project = cplusminus
+
+using version
+using config
+using test
+using install
+using dist

build/root.build

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+# Uncomment to suppress warnings coming from external libraries.
+#
+#cxx.internal.scope = current
+
+cxx.std = latest
+
+using cxx
+
+hxx{*}: extension = hxx
+ixx{*}: extension = ixx
+txx{*}: extension = txx
+cxx{*}: extension = cxx
+
+# Assume headers are importable unless stated otherwise.
+#
+hxx{*}: cxx.importable = true
+
+# The test target for cross-testing (running tests under Wine, etc).
+#
+test.target = $cxx.target