If you can't or won't spend time in fixing compline/d_gconvert.U ...

If I can fix it, then I will undertake to do so.
At the moment, however, I have no idea how d_gconvert.U fits into "the scheme of things".
AFAICS, it's not part of the perl source.

... but I am always scared of interpolation when requiring quotes

I bothers me a bit, too. Although the configure option that I used seems to have done the trick, it wasn't exactly noiseless:

In file included from sv.c:32:0:
sv.c: In function ‘Perl_sv_vcatpvfn_flags’:
config.h:909:39: warning: ‘%.*g’ directive writing between 1 and 133 b
+ytes into a region of size 127 [-Wformat-overflow=]
 #define Gconvert(x,n,t,b) sprintf((b),"%.*g",(n),(x))
                                       ^
perl.h:6791:13: note: in definition of macro ‘WITH_LC_NUMERIC_SET_TO_N
+EEDED_IN’
             block;                                                   
+       \
             ^~~~~
sv.c:48:5: note: in expansion of macro ‘PERL_UNUSED_RESULT’
     PERL_UNUSED_RESULT(Gconvert((NV)(nv), (int)ndig, 0, buffer))
     ^~~~~~~~~~~~~~~~~~
sv.c:48:24: note: in expansion of macro ‘Gconvert’
     PERL_UNUSED_RESULT(Gconvert((NV)(nv), (int)ndig, 0, buffer))
                        ^~~~~~~~
sv.c:13118:21: note: in expansion of macro ‘SNPRINTF_G’
                     SNPRINTF_G(fv, ebuf, sizeof(ebuf), precis)
                     ^
config.h:909:39: note: assuming directive output of 132 bytes
 #define Gconvert(x,n,t,b) sprintf((b),"%.*g",(n),(x))
                                       ^
perl.h:6791:13: note: in definition of macro ‘WITH_LC_NUMERIC_SET_TO_N
+EEDED_IN’
             block;                                                   
+       \
             ^~~~~
sv.c:48:5: note: in expansion of macro ‘PERL_UNUSED_RESULT’
     PERL_UNUSED_RESULT(Gconvert((NV)(nv), (int)ndig, 0, buffer))
     ^~~~~~~~~~~~~~~~~~
sv.c:48:24: note: in expansion of macro ‘Gconvert’
     PERL_UNUSED_RESULT(Gconvert((NV)(nv), (int)ndig, 0, buffer))
                        ^~~~~~~~
sv.c:13118:21: note: in expansion of macro ‘SNPRINTF_G’
                     SNPRINTF_G(fv, ebuf, sizeof(ebuf), precis)
                     ^
In file included from /usr/include/stdio.h:862:0,
                 from perlio.h:41,
                 from iperlsys.h:50,
                 from perl.h:3934,
                 from sv.c:32:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:33:10: note: ‘__builtin___
+sprintf_chk’ output between 2 and 134 bytes into a destination of siz
+e 127
   return __builtin___sprintf_chk (__s, __USE_FORTIFY_LEVEL - 1,
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       __bos (__s), __fmt, __va_arg_pack ());
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from sv.c:32:0:
config.h:909:39: warning: ‘%.*g’ directive writing between 1 and 133 b
+ytes into a region of size 127 [-Wformat-overflow=]
 #define Gconvert(x,n,t,b) sprintf((b),"%.*g",(n),(x))
                                       ^
perl.h:6791:13: note: in definition of macro ‘WITH_LC_NUMERIC_SET_TO_N
+EEDED_IN’
             block;                                                   
+       \
             ^~~~~
sv.c:48:5: note: in expansion of macro ‘PERL_UNUSED_RESULT’
     PERL_UNUSED_RESULT(Gconvert((NV)(nv), (int)ndig, 0, buffer))
     ^~~~~~~~~~~~~~~~~~
sv.c:48:24: note: in expansion of macro ‘Gconvert’
     PERL_UNUSED_RESULT(Gconvert((NV)(nv), (int)ndig, 0, buffer))
                        ^~~~~~~~
sv.c:13118:21: note: in expansion of macro ‘SNPRINTF_G’
                     SNPRINTF_G(fv, ebuf, sizeof(ebuf), precis)
                     ^
config.h:909:39: note: assuming directive output of 132 bytes
 #define Gconvert(x,n,t,b) sprintf((b),"%.*g",(n),(x))
                                       ^
perl.h:6791:13: note: in definition of macro ‘WITH_LC_NUMERIC_SET_TO_N
+EEDED_IN’
             block;                                                   
+       \
             ^~~~~
sv.c:48:5: note: in expansion of macro ‘PERL_UNUSED_RESULT’
     PERL_UNUSED_RESULT(Gconvert((NV)(nv), (int)ndig, 0, buffer))
     ^~~~~~~~~~~~~~~~~~
sv.c:48:24: note: in expansion of macro ‘Gconvert’
     PERL_UNUSED_RESULT(Gconvert((NV)(nv), (int)ndig, 0, buffer))
                        ^~~~~~~~
sv.c:13118:21: note: in expansion of macro ‘SNPRINTF_G’
                     SNPRINTF_G(fv, ebuf, sizeof(ebuf), precis)
                     ^
In file included from /usr/include/stdio.h:862:0,
                 from perlio.h:41,
                 from iperlsys.h:50,
                 from perl.h:3934,
                 from sv.c:32:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:33:10: note: ‘__builtin___
+sprintf_chk’ output between 2 and 134 bytes into a destination of siz
+e 127
   return __builtin___sprintf_chk (__s, __USE_FORTIFY_LEVEL - 1,
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       __bos (__s), __fmt, __va_arg_pack ());
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[download]

Of course, I probably should at least run make test before I start getting too excited about how successful it has been ;-)

Cheers,
Rob