Makefile: increased gcc warning verification
asprintf() and vsprintf() functions need _GNU_SOURCE defined
buffer.c: avoiding comparsion among signed and unsigned integers
`mode' is a global variable, changing it to `mymode' in order to avoid
shadowing of global var.
display.c: snprintf may return negative values, while sizeof()
returns size_t. Casting sizeof() return to `int' to avoid a
warning when comparing
echo.c: avoiding comparsions between signed and unsigned vars
Merge branch 'ricardomaraschini-master'

Also:
- Removed _GNU_SOURCE from makefile;
- Added _GNU_SOURCE in some files to remove warning;
- Fixed compilation on some Debian systems by moving LDFLAGS in compilation commad-line;