BuildSystem: Avoid obsolete warning with libcurl.m4 on autoconf.

Updated the macro-set autoconf/libcurl.m4 to its latest upstream version. This
avoids a warning about an obsolete macro on AC_HELP_STRING, because Asterisk is
using AS_HELP_STRING everywhere else already.

ASTERISK-26046

Change-Id: I8299faf504ceaeee3e39930c59293809e116c631