allow to build gcc-4.5-source package only

same patch should apply to gcc-4.4

Newer version which uses BACKPORT variable.

but this still builds a gcc-4.5-base, which overwrites the one from the distribution?

No, it prepares everything needed for gcc-4.5-base package, but does not create such one. I can provide build log.

This bug was fixed in the package gcc-4.5 - 4.5.1-12ubuntu1

---------------
gcc-4.5 (4.5.1-12ubuntu1) natty; urgency=low

  * Merge with Debian, remaining changes:
    - Build from the upstream tarball, including GFDL documents.

gcc-4.5 (4.5.1-12) experimental; urgency=low

  * Update to SVN 20101210 (r167686) from the gcc-4_5-branch (4.5.2 rc).
    - Fix PR fortran/45742, PR tree-optimization/46498, PR target/45807,
      PR target/44266, PR rtl-optimization/46315, PR tree-optimization/44545,
      PR tree-optimization/46491, PR rtl-optimization/46571, PR target/31100,
      PR c/46547, PR fortran/46638, PR tree-optimization/46675, PR debug/46258,
      PR ada/40777, PR fortran/46753, PR libgomp/45240, PR target/43897,
      PR rtl-optimization/46777, PR rtl-optimization/46614,
      PR middle-end/46629, PR middle-end/46499, PR tree-optimization/44676,
      PR target/45870, PR middle-end/46534, PR rtl-optimization/46440,
      PR c++/46401, PR tree-optimization/46806, PR tree-optimization/46663,
      PR debug/46123, PR middle-end/46651, PR c++/46538, PR c++/46058,
      PR fortran/46794, PR target/45693, PR libgcj/42986.

  [ Matthias Klose ]
  * Use lib instead of lib64 as the 64bit system dir on biarch
    architectures defaulting to 64bit. Closes: #603597.
  * Fix powerpc and s390 builds when biarch is disabled.
  * Backport PR bootstrap/44768, miscompilation of dpkg on ARM
    with -O2 (Chung-Lin Tang). LP: #674146.
  * Update libgcc2 symbols file. Closes: #602099.
  * Update the Linaro support to the 4.5 2010.12 release.

  [ Marcin Juszkiewicz ]
  * Do not depend on target mpfr and zlib -dev packages for cross builds.
    LP: #676027.
  * Apply ARM target configure options for stage1/stage2 cross builds.
    LP: #684625.
  * Allow to build only the -source package, and include the version
    in the package name. LP: #682333.

  [ Konstantinos Margaritis ]
  * Add support for new target architecture `armhf'. Closes: #603948.

gcc-4.5 (4.5.1-11) experimental; urgency=low

  * Update to SVN 20101114 (r166728) from the gcc-4_5-branch.
    - Fix PR fortran/45742.
  * Don't hardcode debian/patches when referencing patches. Closes: #600502.
 -- Matthias Klose <email address hidden> Fri, 10 Dec 2010 20:19:12 +0100