Link against system bcc

Previously, bpftrace was clone bcc from github at build time, building
a version of bcc, then linking against the built bcc. This is a no-go
for many packaging systems including Fedora, openSUSE, and the internal
Facebook one.

This patch requires bcc and bcc development headers be installed to
build bpftrace and removes the build time downloading behavior.
Also simplifies some of the header search logic.

Test plan:
```
$ ./build/tests/bpftrace_test
...
$ sudo make -C build runtime-tests
...

$ cat ~/scratch/bpftrace/path.bt

kprobe:vfs_open
{
    printf("open path: %s\n", str(((path *)arg0)->dentry->d_name.name));
}

$ sudo ./build/src/bpftrace ~/scratch/bpftrace/path.bt
[sudo] password for dlxu:
Attaching 1 probe...
open path: stat
open path: user-745673103.journal
open path: user-1001.journal
open path: user-745673103.journal
open path: user-1001.journal
open path: user-745673103.journal
open path: user-1001.journal
open path: iio:device0
open path: stat
...