Adapt to RPM 4.20

...which builds the spec file's code in a dedicated directory now, see
<rpm-software-management/rpm#2078> "RFE: introduce an
rpm-controlled per-build directory to builds", and no longer in the directory
from which rpm is invoked.  So invent some libo_start_dir parameter to
communicate that directory into the rpm build.

Change-Id: I28268837caafe3b892f76e936dfa747fe13fb9bd
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/173779
Reviewed-by: Stephan Bergmann <stephan.bergmann@allotropia.de>
Tested-by: Jenkins

sysui/CustomTarget_rpm.mk

@@ -36,6 +36,7 @@ $(rpm_WORKDIR)/$(1)/$(1)$(PKGVERSIONSHORT)-$(2)-menus-$(PKGVERSION)-$(LIBO_VERSI
  --define "version $(PKGVERSION)" \
  --define "release $(LIBO_VERSION_PATCH)" \
  --define "__debug_install_post %nil" \
+ --define "libo_start_dir $(gb_CustomTarget_workdir)/sysui/share/$(1)" \
  , $$@.log \
  )
 endef

sysui/desktop/freedesktop/freedesktop-menus.spec

@@ -60,7 +60,7 @@ export KDEMAINDIR=/usr
 export PREFIXDIR=/usr
 export BINDIR=/usr/bin
 
-./create_tree.sh
+(cd %libo_start_dir && ./create_tree.sh)
 
 cd $RPM_BUILD_ROOT