Error if _GLIBCXX_USE_CXX11_ABI is not defined

Within the `__GXX_ABI_VERSION` block this should always be defined,
guard against unexpected defines and make the error obvious.

Author: cryos

include/pybind11/conduit/pybind11_platform_abi_id.h

@@ -75,8 +75,10 @@
 #    elif defined(_LIBCPP_ABI_VERSION) // https://libcxx.llvm.org/DesignDocs/ABIVersioning.html
 #        define PYBIND11_BUILD_ABI "_abi" PYBIND11_PLATFORM_ABI_ID_TOSTRING(_LIBCPP_ABI_VERSION)
 #    elif defined(__GXX_ABI_VERSION)
-#        if __GXX_ABI_VERSION >= 1002 && __GXX_ABI_VERSION < 2000                                 \
-            && defined(_GLIBCXX_USE_CXX11_ABI)
+#        if __GXX_ABI_VERSION >= 1002 && __GXX_ABI_VERSION < 2000
+#            if !defined(_GLIBCXX_USE_CXX11_ABI)
+#                error "UNEXPECTED: _GLIBCXX_USE_CXX11_ABI not defined"
+#            endif
 #            define PYBIND11_BUILD_ABI                                                            \
                 "_gxx_abi_1xxx_usecxx11_" PYBIND11_PLATFORM_ABI_ID_TOSTRING(_GLIBCXX_USE_CXX11_ABI)
 #        else