Building an rpm with rpmbuild fails with missing dependency errors

Issue

• packages are installed, but rpmbuild fails with missing dependency errors, e.g.,

# rpmbuild -bb rpmbuild/SPECS/ruby19.spec > /tmp/ruby_build.txt 2>&1
# grep -i libyaml /tmp/ruby_build.txt 
libyaml is missing. Please install libyaml.
# grep BuildRequires rpmbuild/SPECS/ruby19.spec
BuildRequires:  readline libyaml libyaml-devel readline-devel ncurses ncurses-devel gdbm gdbm-devel glibc-devel tcl-devel gcc unzip openssl-devel db4-devel byacc make libffi-devel
# yum list installed | grep -i libyaml
libyaml.ppc                       0.0.1-1.el5.rf            installed           
libyaml-devel.ppc                 0.0.1-1.el5.rf            installed           
# rpm -qa | grep -i libyaml
libyaml-0.0.1-1.el5.rf.ppc
libyaml-devel-0.0.1-1.el5.rf.ppc

• Why does rpmbuild say a library is not installed even though it is?
• rpmbuild gives this warning message during rpmbuild:

/usr/bin/ld: skipping incompatible /usr/lib/gcc/ppc64-redhat-linux/4.4.7/../../../libyaml.so when searching for -lyaml