Do not compile .c and .S files in D mode

[jpf91]

Fixes https://bugzilla.gdcproject.org/show_bug.cgi?id=244

The .d.lo: rule is already responsible for compiling D sources. The LIBTOOLFLAGS=--tag=D is only required to make libtool link using gdc. This is useful to record the library dependencies (pthread,...) into the shared libraries. But instead of using the tag which then also applies to the sources we can also just provide a custom link command. We can remove all those custom link commands if we properly configure all dependencies (i.e. pull 240).

Note: We have to keep the empty LIBTOOLFLAGS to avoid these error messages:

src/Makefile.am: object `inflate.$(OBJEXT)' created both with libtool and without

BTW: The best error message ever:

xgcc: error: unrecognized command line option '-nophoboslib'; did you mean '-nophoboslib'?

[ibuclaw]

Thanks.