Is it possible to make `xrdb` ignore single quotes on a commented line?

This seems to be due to a change in the default behavior of GNU cpp, which xrdb uses as its default preprocessor. Specifically, according to The C Preprocessor: 10.1 Traditional lexical analysis:

Generally speaking, in traditional mode an opening quote need not have a matching closing quote.

However cpp provides a command line option to operate in traditional mode:

-traditional
-traditional-cpp
    Try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO 
    C preprocessors. See Traditional Mode.

while xrdb allows the preprocessor to be defined explicitly on its command line:

-cpp filename
       This  option  specifies the pathname of the C preprocessor pro‐
       gram to be used.  Although xrdb was designed to  use  CPP,  any
       program  that  acts  as a filter and accepts the -D, -I, and -U
       options may be used.

Hence it should be possible to suppress the warning by using

xrdb -cpp "/usr/bin/cpp -traditional-cpp"  ~/.Xresources

or

xrdb -cpp "/usr/bin/cpp -traditional"  ~/.Xresources