debian/rules creates a file /usr/include/pike/7.6.112/specs containing various build parameters, such as CPPFLAGS and configure_args. I don't know if any other package uses it.
That file is (supposed to be) used by pike -x module.
Oh, now I see. I'll take a closer look at the differences compared to the ordinary one.
The original generated specs file looks as follows:
| CC=/usr/include/pike/7.6.112/smartlink gcc
debian/specs.in simply sets CC=gcc. Isn't smartlink used for a reason? It's even patched.
| CFLAGS= -Wa,--execstack -mcpu=i686
debian/specs.in simply sets CFLAGS=-O2 -DDEBIAN, but I see no reason to override it and move -DDEBIAN from CPPFLAGS. The -mcpu makes me a bit worried that the build system optimizes too much. AFAICS that can only be disabled by setting CFLAGS to something. But what? And where did -O2 go? --without-copt was not used. This line in src/configure.in looks like a bug (but it shouldn't affect anything):
AC_ARG_WITH(copt, MY_DESCR([--without-mmx], [disable MMX usage]), [], [with_mmx=])
| LDFLAGS= -L/tmp/buildd/pike7.6-7.6.112/build/linux-2.6.18-proffe-4-i686/bundles/lib -z execstack -R/usr/local/lib/. -L/usr/local/lib/. | CPPFLAGS= -DDEBIAN -I/tmp/buildd/pike7.6-7.6.112/build/linux-2.6.18-proffe-4-i686/bundles/include -I/usr/local/include
I think --without-bundles should help here. debian/specs.in also excludes the /usr/local/ directories and redundantly includes -L/usr/lib and -I/usr/include, the obsolete /usr/X11R6/lib, and /usr/include/pike/$VERSION. I don't think the Debian package should have to add that last one.
| CPP=gcc -E | LDSHARED=gcc -shared | configure_args='--without-rtldebug' '--without-cdebug' '--without-debug' '--with-cflags=' '--with-cppflags=-DDEBIAN' '--with-bignums' '--with-gmp' '--with-poll' '--with-zlib' '--with-freetype' '--without-ttflib' '--with-libnettle' '--without-sybase' '--without-java' '--with-odbc' '--with-sane' '--with-postgres' '--with-postgres-include-dir=/usr/include/postgresql' '--with-libpq-dir=/usr/lib' '--with-perl' '--without-ffmpeg' '--without-fftw' '--without-libpdf' '--without-libpanda' '--without-GTK' '--without-GTK2' '--with-machine-code' '--with-security' 'CC=gcc' 'CFLAGS='
No differences here, except for quoting style.