[D-runtime] Time for druntime for Linux64 to default to a shared lib

[Walter Bright]

Thanks to the efforts of Martin Nowak (dawgfoto) I have been able to build 
druntime as a shared library under Linux64 and get it to successfully run the 
test suite.

Therefore, it's now time to make this now the default. This requires some 
changes to the makefiles for druntime, phobos, and some tweaks to dmd.

druntime:
     PIC needs to default to -fPIC
     dll is now the default build

phobos:
     DRUNTIME needs to default to being nothing

dmd:
     prepend "-ldruntime-linux64" to the list of standard libraries passed to 
the linker

dmd.conf:
     needs -L-Lpath/druntime/lib added to DFLAGS

and,
     export LD_LIBRARY_PATH=path/druntime/lib
needs to be executed by the user.

Once this is all working for Linux64, we can extend it to the other platforms.