build: Force -fsigned-char on platforms where the default for char is unsigned

gcc on the ARM platform defaults 'char' to 'unsigned char' whereas Intel and
SPARC default to 'signed char'.  This is only an issue in the rare cases where
negative values are assigned to a 'char' but this this patch insures
compatibility by detecting platforms that default to 'unsigned' and adding an
'-fsigned-char' flag to _ASTCFLAGS.

If compiling for ARM (native or cross-compile) be sure to run ./bootstrap.sh
and ./configure to regenerate the build files.  You shouldn't have to do this
for Intel or SPARC.

Tested-by: George Joseph

Review: https://reviewboard.asterisk.org/r/4091/
........

Merged revisions 425964 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@425965 f38db490-d61c-443f-a65b-d21fe96a405b