macros: add %_cross_buildsrcdir

Twoliter mounts sources for the current workspace into the SDK build
environment at `/home/builder/rpmbuild/BUILD/sources`, which packages
can refer to via the %_builddir macro - e.g. "%{_builddir}/sources" -
to avoid hard-coding assumptions about the SDK environment, such as
the user account that executes the build.

In RPM 4.20, the %_builddir macro now points to a package-specific
directory rather than the top-level build directory, which breaks
this approach.

Define %_cross_buildsrcdir as a replacement for %_builddir which will
always point to the path in the SDK where Twoliter mounts the sources
from the current workspace.

Signed-off-by: Ben Cressey <[email protected]>

Author: bcressey

macros/shared

@@ -23,6 +23,7 @@
 %_cross_ldflags -Wl,-z,relro -Wl,-z,now -Wl,-Bsymbolic-functions
 %_cross_c_link_args '-Wl,-z,relro', '-Wl,-z,now', '-Wl,-Bsymbolic-functions'
 
+%_cross_buildsrcdir %{_topdir}/BUILD/sources
 %_cross_bootdir /boot
 %_cross_bootconfigdir %{_cross_bootdir}/boot-config.d
 %_cross_lib lib