how ${buildroot} is created automatically during rpmbuild? - centos6

Question

I want to create the binary rpm files from generated binary packages in centos 6.4 and noticed in one of my build system the ${buildroot} is created automatically, I want to know how it can be achieved.
The sample wget.spec looks like below. detail spec is in my gist https://gist.github.com/larrycai/1ffe134bce1860f1cf87
%install
tar xf ~/wget.tar.gz -C %{buildroot}/
%files
%defattr(-,root,root)
/usr/bin/wget
The wget.tar.gz can be created tar -zcvf ~/wget.tar.gz /usr/bin/wget
When I build it using rpmbuild -bb wget.spec, I got the
+ rpmbuild -bb wget.spec
Executing(%prep): /bin/sh -e /var/tmp/rpm-tmp.VNDZg4
+ umask 022
+ cd /home/larry/rpmbuild/BUILD
+ LANG=C
+ export LANG
+ unset DISPLAY
+ exit 0
Executing(%install): /bin/sh -e /var/tmp/rpm-tmp.S8koPW
+ umask 022
+ cd /home/larry/rpmbuild/BUILD
+ '[' /home/larry/rpmbuild/BUILDROOT/wget-1.12-1.x86_64 '!=' / ']'
+ rm -rf /home/larry/rpmbuild/BUILDROOT/wget-1.12-1.x86_64
++ dirname /home/larry/rpmbuild/BUILDROOT/wget-1.12-1.x86_64
+ mkdir -p /home/larry/rpmbuild/BUILDROOT
+ mkdir /home/larry/rpmbuild/BUILDROOT/wget-1.12-1.x86_64
+ LANG=C
+ export LANG
+ unset DISPLAY
+ tar xf /home/larry/wget.tar.gz -C /home/larry/rpmbuild/BUILDROOT/wget-1.12-1.x86_64/
+ /usr/lib/rpm/check-buildroot
... (it is successful)
I wonder why line mkdir /home/larry/rpmbuild/BUILDROOT/wget-1.12-1.x86_64 is executed automatically.
Are there any script or rpmmacros is enabled for this ?

Answer 1

The main reason is that build machines installed redhat-rpm-config package, the detail list below
This macro setting is in /usr/lib/rpm/redhat/macros
%__spec_install_pre %{___build_pre}\
[ "$RPM_BUILD_ROOT" != "/" ] && rm -rf "${RPM_BUILD_ROOT}"\
mkdir -p `dirname "$RPM_BUILD_ROOT"`\
mkdir "$RPM_BUILD_ROOT"\
%{nil}
We can either install this package or put this macros into ~/.rpmmacros
I find it by running command rpmbuild --showrc can see that mkdir code segment is there, so I start to check whether there is special package is installed.