perf script: Fix Python support when no libtraceevent

perf script: Fix Python support when no libtraceevent

Python scripting can be used without libtraceevent. In particular,
scripting for Intel PT does not use tracepoints, and so does not need
libtraceevent support.

Alter the build and employ conditional compilation to allow Python
scripting without libtraceevent.

Example:

 Before:

    $ ldd `which perf` | grep -i python
    $ ldd `which perf` | grep -i libtraceevent
    $ perf record -e intel_pt//u uname
    Linux
    [ perf record: Woken up 1 times to write data ]
    [ perf record: Captured and wrote 0.031 MB perf.data ]
    $ perf script intel-pt-events.py |& head -3
      Error: Couldn't find script `intel-pt-events.py'

     See perf script -l for available scripts.

 After:

    $ ldd `which perf` | grep -i python
            libpython3.10.so.1.0 => /lib/x86_64-linux-gnu/libpython3.10.so.1.0 (0x00007f4bac400000)
    $ ldd `which perf` | grep -i libtraceevent
    $ perf script intel-pt-events.py | head
    Intel PT Branch Trace, Power Events, Event Trace and PTWRITE
         Switch In    8021/8021  [000]     11234.097713404     0/0
           perf-exec  8021/8021  [000]     11234.098041726       psb                        offset: 0x0                0 [unknown] ([unknown])
           perf-exec  8021/8021  [000]     11234.098041726       cbr                         45  freq: 4505 MHz  (161%)                0 [unknown] ([unknown])
               uname  8021/8021  [000]     11234.098082170  branches:uH  tr strt                              0 [unknown] ([unknown]) => 7f3a8b9422b0 _start+0x0 (/usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2)
               uname  8021/8021  [000]     11234.098082379  branches:uH  tr end                    7f3a8b9422b0 _start+0x0 (/usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2) => 0 [unknown] ([unknown])
               uname  8021/8021  [000]     11234.098083629  branches:uH  tr strt                              0 [unknown] ([unknown]) => 7f3a8b9422b0 _start+0x0 (/usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2)
               uname  8021/8021  [000]     11234.098083629  branches:uH  call                      7f3a8b9422b3 _start+0x3 (/usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2) => 7f3a8b943050 _dl_start+0x0 (/usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2)
               uname  8021/8021  [000]     11234.098083837  branches:uH  tr end                    7f3a8b943060 _dl_start+0x10 (/usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2) => 0 [unknown] ([unknown])  IPC: 0.01 (9/938)
               uname  8021/8021  [000]     11234.098084670  branches:uH  tr strt                              0 [unknown] ([unknown]) => 7f3a8b943060 _dl_start+0x10 (/usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2)

Fixes: 378ef0f5d9d7f465 ("perf build: Use libtraceevent from the system")
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20230315084321.14563-1-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>