configure.ac: fix readline static build

Use pkg-config to retrieve readline dependencies such as ncurses to
avoid the following static build when readline is built with ncurses
support (which is raised since version 1.8.19 and
https://github.com/ipmitool/ipmitool/commit/63dd71c39c9bf522ad3a785dd001737a285458eb):

** Unable to build Solaris 9 x86 IPMI interface support!
checking for library containing readline... no
configure: error: ** Unable to find readline required by ipmishell.

[...]

configure:15125: /tmp/instance-0/output-1/host/bin/armeb-buildroot-linux-musleabi-gcc -o conftest -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -O0 -g0  -static -Wall -Wextra -std=gnu11 -pedantic -Wformat -Wformat-nonliteral -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -static conftest.c -lreadline   -lcrypto >&5
/tmp/instance-0/output-1/host/lib/gcc/armeb-buildroot-linux-musleabi/11.3.0/../../../../armeb-buildroot-linux-musleabi/bin/ld: /tmp/instance-0/output-1/host/armeb-buildroot-linux-musleabi/sysroot/usr/lib/libreadline.a(display.o): in function `_rl_move_cursor_relative':
display.c:(.text+0x80fc): undefined reference to `tputs'

Fixes:
 - http://autobuild.buildroot.org/results/dabc6a4f49d464c129ac6bc3710011678142fcbe

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>