[v3] fixup --enable-cxx-flags

[Benjamin De Kosnik]

... found while working on arm-eabisim cross, using --enable-cxx-flags
was not working as AM_CXXFLAGS was being over-ridden by CXXFLAGS.
(Despite the comments warning about this.)

Fixed.

Also patchlet for the last commit, forgot to edit PARALELL_FLAGS, so
--disable-thread compiles were failing. 

-benjamin

tested x86/linux
tested x86/linux --enable-cxx-flags="-g0"
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