CMakeLists.txt: use pkg-config to find readline
CMakeLists.txt: use pkg-config to find readline

Use pkg_check_modules to find readline dependencies such as ncurses and
fallback on current mechanism.

This will fix the following build failure when building statically:

/home/buildroot/autobuild/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabi/8.3.0/../../../../arm-buildroot-linux-uclibcgnueabi/bin/ld: /home/buildroot/autobuild/instance-1/output-1/host/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libreadline.a(display.o): in function `cr':
display.c:(.text+0x1a0): undefined reference to `tputs'

Fixes:
 - http://autobuild.buildroot.org/results/88609eefe55af2ca50d43e17d3424b923528b07a

Signed-off-by: Fabrice Fontaine <7d88633730b8b85c1516e612893df72eae9cd341@gmail.com>

FluidSynth/fluidsynth,FluidSynth/fluidsynth,FluidSynth/fluidsynth,FluidSynth/fluidsynth