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the other (and better way) is to tell your compiler the C++ comments are ok, I use
xlC for my C compiler so I do it this way:

export CFLAGS="-qcpluscmt"

run ./configure
then make

from help for xlC:

-qcpluscmt | -qnocpluscmt
Permits the usage of "//" to introduce a comment
that lasts until the end of the current source
line, as in C++. The default is -qcpluscmt when
you invoke the compiler with xlc, xlc_r, cc, or
cc_r, or when -qlanglvl is set to stdc99 or extc99.
Otherwise, the default setting is -qnocpluscmt.