Force base dir macros to arbitrary values, derived as relative to base ones.
Now redefining _prefix affects _{bin,sbin,lib,libexec,include,data,dataroot,info,man,doc}dir values.

--- rpm-4.16.1.3/platform.in.orig	2021-03-28 18:12:24.786830780 +0200
+++ rpm-4.16.1.3/platform.in	2021-03-28 18:23:20.670665669 +0200
@@ -25,23 +25,23 @@
 #==============================================================================
 # ---- configure macros.
 #
-%_prefix		@prefix@
-%_exec_prefix		@exec_prefix@
-%_bindir		@bindir@
-%_sbindir		@sbindir@
-%_libexecdir		@libexecdir@
-%_datarootdir		@datarootdir@
-%_datadir		@datadir@
-%_sysconfdir		@sysconfdir@
-%_sharedstatedir	@sharedstatedir@
-%_localstatedir		@localstatedir@
+%_prefix		/usr
+%_exec_prefix		%{_prefix}
+%_bindir		%{_prefix}/bin
+%_sbindir		%{_prefix}/sbin
+%_libexecdir		%{_prefix}/libexec
+%_datarootdir		%{_prefix}/share
+%_datadir		%{_datarootdir}
+%_sysconfdir		/etc
+%_sharedstatedir	/var/lib
+%_localstatedir		/var
 %_lib			@LIB@
-%_libdir		@exec_prefix@/@LIB@
-%_includedir		@includedir@
-%_oldincludedir		@oldincludedir@
-%_infodir		@infodir@
-%_mandir		@mandir@
-%_initddir		%{_sysconfdir}/rc.d/init.d
+%_libdir		%{_exec_prefix}/@LIB@
+%_includedir		%{_prefix}/include
+%_oldincludedir		%{_prefix}/include
+%_infodir		%{_datarootdir}/info
+%_mandir		%{_datarootdir}/man
+%_initddir		/etc/rc.d/init.d
 # Deprecated misspelling, present for backwards compatibility.
 %_initrddir		%{_initddir}
 %_rundir		@RUNDIR@