libtor: make building on Linux more robust

[bassosimone]

• pass --disable-system-torrc to ./configure to avoid picking up a system torrc in integration tests (which is also more correct to do on mobile devices anyway);
• pass --disable-seccomp to ./configure to avoid picking up libseccomp, which we don't have on mobile anyway;
• patch the build system to avoid trying to use libcap2, given that we don't have that on mobile anyway.

Based on a conversation with @ainghazal during the review of ooni/probe-cli#1445.

[bassosimone]

% cat LOGS.txt|grep cap_init
checking for cap_init in -lcap... no
checking for cap_init in -lcap... no
checking for cap_init in -lcap... no
checking for cap_init in -lcap... no

% cat LOGS.txt|grep seccomp
checking seccomp.h usability... no
checking seccomp.h presence... no
checking for seccomp.h... no
checking for library containing seccomp_init... no
  libseccomp (--disable-seccomp):                                yes
checking seccomp.h usability... no
checking seccomp.h presence... no
checking for seccomp.h... no
checking for library containing seccomp_init... no
  libseccomp (--disable-seccomp):                                yes
checking seccomp.h usability... no
checking seccomp.h presence... no
checking for seccomp.h... no
checking for library containing seccomp_init... no
  libseccomp (--disable-seccomp):                                yes
checking seccomp.h usability... no
checking seccomp.h presence... no
checking for seccomp.h... no
checking for library containing seccomp_init... no
  libseccomp (--disable-seccomp):                                yes

This is the Android build.

[bassosimone]

% cat LOGS.txt|grep cap_init
checking for cap_init in -lcap... no
checking for cap_init in -lcap... no
checking for cap_init in -lcap... no

% cat LOG.txt|grep seccomp
checking seccomp.h usability... no
checking seccomp.h presence... no
checking for seccomp.h... no
checking for library containing seccomp_init... no
  libseccomp (--disable-seccomp):                                yes
checking seccomp.h usability... no
checking seccomp.h presence... no
checking for seccomp.h... no
checking for library containing seccomp_init... no
  libseccomp (--disable-seccomp):                                yes
checking seccomp.h usability... no
checking seccomp.h presence... no
checking for seccomp.h... no
checking for library containing seccomp_init... no
  libseccomp (--disable-seccomp):                                yes

and this is iOS.

[ainghazal]

fwiw, I can confirm the tests mentioned in ooni/probe-cli#1445 can run successfully on an arch system now.