Add support for nixos

[ragnard]

Two changes that enables bcc to build and work nicely on nixos:

• add -I/build/include/generated when calculating cflags. This is where autoconf.h with #define CONFIG_BPF_SYSCALL 1 lives so it is critical. This appears to be true not only for nixos, but I checked Ubuntu 14.04.3 LTS too and the location was the same.

• bcc quite reasonably assumes that the kernel modules dir is /lib/modules/, encoded using #define KERNEL_MODULES_DIR "/lib/modules". This is not true for nixos however. The proposed patch:

  • introduces a BCC_KERNEL_MODULES_DIR cmake flag
  • defaults that flag to /lib/modules, so no breaking change
  • sets KERNEL_MODULES_DIR based on that flag.

  This allows us to override KERNEL_MODULES_DIR when building bcc for nixos.

[4ast]

lgtm

[drzaeus77]

Thanks!