Support cross-compiling for uClibc targets

Disable backtrace support in src/config.h if uClibc is detected.

Tested on the following platforms:

- sh4-linux-uclibc (GCC 4.1.1, uClibc 0.9.29)
- mipsel-linux-uclibc (GCC 4.4.5, uClibc 0.9.29)

To cross-compile:

    $ make CC=mipsel-linux-gcc MALLOC=libc

Signed-off-by: Michael Steinert <mike.steinert@gmail.com>