MFC r312506:

Use SRCTOP-relative paths to other directories instead of .CURDIR-relative ones

This simplifies pathing in make/displayed output

MFC r260334:

Split the last gcc-specific flags off into CFLAGS.gcc. This also
removes the need to use -Qunused-arguments for clang throughout the
tree.

MFC r260369:

Apply band-aid for 32-bit compat libs failures after r260334: put back
-Qunused-arguments for clang for now, until I can figure out a way to
make it unneeded in all scenarios. Sorry about the breakage.

MFC r260334:

Split the last gcc-specific flags off into CFLAGS.gcc. This also
removes the need to use -Qunused-arguments for clang throughout the
tree.

MFC r260369:

Apply band-aid for 32-bit compat libs failures after r260334: put back
-Qunused-arguments for clang for now, until I can figure out a way to
make it unneeded in all scenarios. Sorry about the breakage.