Avoiding hardcoded paths when static-compiling

[Konstantin Ryabitsev]

Hi, all:

I provide an RPM package called gnupg22-static for those who need to run 
newer versions of GnuPG on CentOS-7 environments (it's stuck on 
gnupg-2.0 there). For compilation, I use the convenient STATIC=1 
mechanism, but there's still the problem that all paths end up being 
hardcoded to the RPM buildroot environment.

The full build command is:
make -f build-aux/speedo.mk STATIC=1 CUSTOM_SWDB=1 INSTALL_PREFIX=.  this-native 

In the RPM context, the INSTALL_PREFIX ends up being inside a buildroot 
location, like so:

/builddir/build/BUILD/gnupg-2.2.17/

However, the final installation of this will be in /opt/gnupg22, which 
means that if a binary needs to call another binary, it will try to 
execute /builddir/build/BUILD/gnupg-2.2.17/bin/foo (and fail).

I can't set INSTALL_PREFIX=/opt/gnupg22, because that will make the RPM 
build fail (it cannot write outside of /builddir), so I need a way to 
tell the binaries during build time that their final install path will 
be different than the path used during build. 

I am able to use gpg and gpgv this way by setting agent-program and 
dirmngr-program config values, but trying to make this work with 
gpg-wks-server fails.

Any pointers on how I can make this work without hardcoding bogus 
build-time paths?

-K