Add this snippet at /usr/lib/rpm/macros

macros

#=======
# this expanded %install scriptlet will create automatically if a directory not exists.
# So, you do not need to worry with rpm .spec file to create directory.

%__spec_install_pre %{___build_pre}\
    [ "$RPM_BUILD_ROOT" != "/" ] && rm -rf "${RPM_BUILD_ROOT}"\
    mkdir -p `dirname "$RPM_BUILD_ROOT"`\
    mkdir "$RPM_BUILD_ROOT"\
%{nil}

Warning:

Updates can overwrite your custom "/usr/lib/rpm/macros" file then I recommend you to check this inline spec if some errors occur at runtime.