Cpp: Set -std=c++11 also if CXXCPP is already set
Cpp: Set -std=c++11 also if CXXCPP is already set

* m4/ax_cxx_compile_stdxx.m4 (AX_CXX_COMPILE_STDCXX): Set CXXCPP if
neccessary.

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This fixes the build with scan-build where CXXCPP is already set but
does not include stdc++11. While this deviates from the
autotools-archive version of the script it does not make sense
to me first to check if stdc++11 needs to be set and then not
set it.

gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme