sim: erc32: fix linking against local readline on modern (ncurses) systems

sim: erc32: fix linking against local readline on modern (ncurses) systems

The current erc32 configure script only searches for -ltermcap to provide
any termcap funcs (which readlines needs).  When building against a local
readline (which is static), we hit link failures like so:
  gcc ...-I/-D flags... -o sis \
    sis.o exec.o erc32.o func.o help.o float.o \
    ../../bfd/libbfd.a ../../opcodes/libopcodes.a \
    ../../libiberty/libiberty.a -lz -lnsl \
    ../../readline/libreadline.a -lm
  ../../readline/libreadline.a(display.o): In function 'cr':
  .../readline/display.c:2486: undefined reference to 'tputs'
  collect2: ld returned 1 exit status
  make[2]: *** [sis] Error 1

Use AC_SEARCH_LIBS from gdb's configure.ac as suggested by Joel Brobecker
to check for additional termcap providers.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>