cacheset: Fix compilation on new GCC in C++98 mode

cacheset: Fix compilation on new GCC in C++98 mode

Since gcc 4.9, the API for erase slightly changed. In
commit 3dddcdf2432e78f37c74d8c76c2c519a8d935ab2 the
existing checks for __cplusplus where changed to
check the gcc version, as the __cplusplus check
did nothing, because gcc 4.8 already provided the
standard value in there.

Fix the code to check for the gcc version in two
more places, and change the existing checks to
use a convenience macro.