Fix gh-687 Build error using gcc 4.6
Fix gh-687 Build error using gcc 4.6

Use -Wl,-export-dynamic to pass the linker flag rather than --export-dynamic,
which (depending on who you believe) either worked by accident, or silently
failed to work at all, on pre-4.6 versions of gcc but gives an error on 4.6

Signed-off-by: Richard Chapman <5900d56196fe29dba8e3ae556d57eae28e9f0195@hpccsystems.com>

afishbeck/HPCC-Platform,afishbeck/HPCC-Platform,afishbeck/HPCC-Platform,afishbeck/HPCC-Platform,afishbeck/HPCC-Platform,afishbeck/HPCC-Platform