fix LINKERFLAG for gdc

this avoids build errors such as:
gdc: error: unrecognized command line option ‘-Xlinker-ldl’
gdc: error: unrecognized command line option ‘-Xlinker-lDerelictGL’
gdc: error: unrecognized command line option ‘-Xlinker-lDerelictIL’
gdc: error: unrecognized command line option ‘-Xlinker-lDerelictUtil’
add object_directories target to ensure output directories exist
convert from const(char)* to string with std.conv.to
Merge pull request #12 from holtrop/gdc-build-fixes

Gdc build fixes