configure.ac: add AM_PROG_AR for automake >= 1.12

fixes:
automake-1.12/am/ltlibrary.am: warning: 'libwebp.la': linking libtool libraries using a non-POSIX
automake-1.12/am/ltlibrary.am: archiver requires 'AM_PROG_AR' in 'configure.ac'

Change-Id: I223f93e5f075aaf23cfefceef55e2ab8eeb34ccd
(cherry picked from commit ed4dc717696ddc791363f2301fb4a142d4ef4633)