changed gcc optimization level from "-O3" to "-O2" for releases

This change was done due to general recommendations and because of
some versions of gcc emitting lots of misleading warnings with "-O3"
related to pointers possibly being null in cases where they surely
cannot be null ("warning: argument 1 null where non-null expected"
for "strcmp(s1, s2)" in src/libgame/misc.c after explicitly checking
that s1 is not null), which makes it harder to recognize "real"
warnings when building the distribution packages.