Fix/libvlc symbolication

Libvlc debug symbols are missing and they are mandatory to analyse some stability issues happening actually on iOS. Two things were found causing this issue :

1. Clang compiler didn't get the -g flag to generate libvlc symbols
2. The libvlc static library target's settings were set to strip them when archive was invoked from the build script, even when Clang compiler generated them as expected

Actually I decided to put the -g flag in the EXTRA_CFLAGS variable in the build script to fix 1., and to change the xcodeproj target's build settings to fix 2.

Maybe 2. can be fixed without changing the target's build settings and by removing the SKIP_INSTALL=no override from the xcodebuild archive script's command, but I bet this setting override has its own purpose there. Any input about this will be appreciated.