r319898 made it possible to override these variables via the
CROSS_TOOLCHAIN_FLAGS setting, but this only worked if one explicitly
specifies these variables there. If, instead, one uses
CROSS_TOOLCHAIN_FLAGS to specify a toolchain file (as our internal
builds do, to point cmake to a checked-in toolchain), the
CMAKE_C(XX)_COMPILER flags would still win over the ones specified by
the toolchain file.
To fix is I make the mere presence of these flags overridable. I do this
by putting them as a default value for the CROSS_TOOLCHAIN_FLAGS
setting, so they can be overridden at cmake configuration time.