MB-8481 Fix strict aliasing issue on gcc 4.4 and below

GNU GCC 4.4.7 and Apple's LLVM'ed GCC 4.2.1 give the following
strict-aliasing warning when compiling with -O2 (which is the
level we use for GNU/Linux releases) or -O3 and -Wstrict-aliasing=2/3
to the compiler:

src/couch_db.c: In function 'find_header_at_pos':
src/couch_db.c:50: warning: dereferencing type-punned pointer will break strict-aliasing rules

In order to get the warning on Apple's 4.2.1, pass -fstrict-aliasing
to the compiler, as the strict-aliasing optimization is not enabled
by default for -O2 level (and above) on GCC versions prior to 4.4.

Currently for GNU/Linux releases at least, we use GCC 4.4 (see
https://github.com/couchbase/voltron/blob/master/README.md).

Change-Id: Ie26ddb68fa3e506b126c99282c2162f35abdf88e
Reviewed-on: http://review.couchbase.org/26994
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>