When buildroot builds arm-linux-uclibcgnueabi toolchain, some libraries get installed into <sysroot>/lib, others - including libstdc++ - into <sysroot>/usr/arm-linux-uclibcgnueabi/lib. If I use such a toolchain as an external toolchain for another buildroot build, buildroot does not find libraries in <sysroot>/usr/arm-linux-uclibcgnueabi/lib. Previous versions of buildroot used -print-file-name option of gcc to locate a library. Current version only checks in <sysroot>/lib folder.
This will be fixed by http://git.buildroot.net/~tpetazzoni/git/buildroot/commit/?h=codesourcery-toolchain-support&id=8f610d63122c205b921408bd3d28af9e64f5d8e6, but I am not going to push this change before 2010.02. It should be part of the next release, 2010.05, where other improvements to external toolchain supports are scheduled.
Fixed by ea2505ee567f716f6b2aeae45ccfe2555c0bcb97 : we now look for libraries in both lib/ and usr/lib in the external toolchain sysroot.