MakeLists.txt: fix static build with curl and libidn2

curl can be statically linked with libidn2, in this case, build fails:
/accts/mlweber1/rclinux/rc-buildroot-test/scripts/instance-0/output/host/m68k-buildroot-linux-uclibc/sysroot/usr/lib/libcurl.a(libcurl_la-url.o): In function `free_idnconverted_hostname.isra.1':
url.c:(.text+0xf4): undefined reference to `idn2_free'

To fix this issue, add a call to pkg_check_modules to retrieve any
needed dependencies

Fixes:
 - http://autobuild.buildroot.org/results/be5893b507d22a23951efeea20c18642742cef5a



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