xerces-c: Use libstdcpp instead of uclibc++

xerces-c: Use libstdcpp instead of uclibc++

Both dependent packages (sumo and libelektra) use (and must use) libstdcpp

Two libc++ cannot be linked against each other at the same time. I believe
this causes xerces-c to not be used even though it is listed as a
dependency.

Took the time to clean up the Makefile. Also replaced -fPIC with -fpic to
eliminate potential issues. Added linker flags for smaller size.

Signed-off-by: Rosen Penev <redacted>