[I'm sure this isn't the first time this has occurred to someone, or even been done -- but couldn't find anything for it in Google ...]
http://et.redhat.com/~rjones/auto-buildrequires/
This tries to find the correct set of BuildRequires automatically when running 'rpmbuild'. Just replace the ordinary rpmbuild command with auto-br-rpmbuild, and it will print out a suggested set of BuildRequires lines at the end. For example:
$ auto-br-rpmbuild -ba mingw32-iconv.spec [...] Checking for unpackaged file(s): /usr/lib/rpm/check-files /home/rjones/rpmbuild/BUILDROOT/mingw32-iconv-1.12-5.fc10.x86_64 Wrote: /home/rjones/rpmbuild/SRPMS/mingw32-iconv-1.12-5.fc10.src.rpm Wrote: /home/rjones/rpmbuild/RPMS/noarch/mingw32-iconv-1.12-5.fc10.noarch.rpm Executing(%clean): /bin/sh -e /var/tmp/rpm-tmp.XpFhTF + umask 022 + cd /home/rjones/rpmbuild/BUILD + cd libiconv-1.12 + rm -rf /home/rjones/rpmbuild/BUILDROOT/mingw32-iconv-1.12-5.fc10.x86_64 + exit 0 BuildRequires: bash = 3.2.29.fc10.x86_64 BuildRequires: binutils = 2.18.50.0.9.7.fc10.x86_64 BuildRequires: ccache = 2.4.13.fc9.x86_64 BuildRequires: coreutils = 6.12.16.fc10.x86_64 BuildRequires: diffutils = 2.8.1.21.fc9.x86_64 BuildRequires: file = 4.26.3.fc10.x86_64 BuildRequires: findutils = 1:4.4.0.1.fc10.x86_64 BuildRequires: gawk = 3.1.5.18.fc10.x86_64 BuildRequires: gcc-gfortran = 4.3.2.6.x86_64 BuildRequires: gettext = 0.17.8.fc10.x86_64 BuildRequires: glibc-common = 2.8.90.14.x86_64 BuildRequires: grep = 2.5.1a.61.fc10.x86_64 BuildRequires: gzip = 1.3.12.7.fc10.x86_64 BuildRequires: make = 1:3.81.14.fc10.x86_64 BuildRequires: mingw32-binutils = 2.18.50_20080109_2.8.fc9.x86_64 BuildRequires: mingw32-filesystem = 34.1.fc9.noarch BuildRequires: mingw32-gcc-c++ = 4.3.2.8.fc9.x86_64 BuildRequires: mingw32-gcc = 4.3.2.8.fc9.x86_64 BuildRequires: mingw32-gettext = 0.17.6.fc9.noarch BuildRequires: mingw32-iconv = 1.12.4.fc9.noarch BuildRequires: mingw32-runtime = 3.15.1.1.fc9.noarch BuildRequires: mingw32-w32api = 3.12.1.fc9.noarch BuildRequires: net-tools = 1.60.91.fc10.x86_64 BuildRequires: sed = 4.1.5.10.fc9.x86_64 BuildRequires: tar = 2:1.20.3.fc10.x86_64
As you can see it's not perfect. It doesn't ignore 'core' packages which are always expected in a mock/koji build. Also because autoconf scripts tend to touch the C++ and Fortran compilers, even when they are not used, it always suggests those packages. Also it doesn't exclude some tools like tar which are touched by rpmbuild itself.
The implementation is a simple LD_PRELOAD script that analyzes open(2) and execve(2) system calls, a Perl script that does the analysis, and some shell scripts to hang it all together.
Rich.