Fix flatpak build

The build system assumes that libpaper is found in the default include
and link paths, but as it is not part of the flatpak runtime, it too is
built into the /app prefix in flatpak builds.

ghostscript.spec

@@ -275,7 +275,7 @@ git commit --all --amend --no-edit > /dev/null
 %configure --disable-compile-inits --without-versioned-path \
            --with-fontpath="%{urw_base35_fontpath}:%{google_droid_fontpath}"
 %endif
-%make_build so
+%make_build so %{?flatpak:XCFLAGS=-I%{_includedir} XTRALIBS=-L%{_libdir}}
 
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