Not cleared for use by IT managers, quality and/or test engineers because of serious hazard of heart attack.

 - Revert [20138]
 - Revert [20139]

Special credits to these commits for:
 - breaking the build system,
 - crashing VLC on every single 64-bits platform,
 - breaking bootstrap with any version of automake that is not the same as the one on Jean-Paul's computer.

Fix one warning for vlc_iconv() here (4th argument), but 2nd argument also issues a warning. What is more dangerous here is there is pointer arithmic done with psz_stringstart. This needs FIXING.

Fix liblua5.1 detection on non-debian system (e.g. mine) that doesn't have lua5.1.pc but do have lua.pc.
Check for lua >= 5.1.