Compiling tcpdump on AIX

• Autoconf works everywhere.

AIX 7.1/POWER7

• Only local libpcap is suitable.
• CMake 3.16.0 does not work.
• GCC 8.3.0 and XL C 12.1.0.0 work.
• System m4 does not work, GNU m4 1.4.17 works.

AIX 7.2/POWER8

• Only local libpcap is suitable.
• GCC 7.2.0 and XL C 13.1.3.6 work.
• System m4 does not work, GNU m4 1.4.17 works.