configure.ac: Check if the compiler supports do { } while (0) macros. This fixes a warning when compiling with g++ 4....

Original commit message from CVS:
* configure.ac:
Check if the compiler supports do { } while (0) macros. This fixes
a warning when compiling with g++ 4.3, resulting in a build failure
because of -Werror.
* ext/mpeg2enc/gstmpeg2encpicturereader.cc:
* ext/mplex/gstmplex.cc:
Include <string.h> for memcpy and friends to fix the build with
gcc 4.3.
* tests/check/Makefile.am:
Remove trailing backslash.