configure: Fix build against ncurses with separate tinfo lib

[Polynomial-C]

ncurses library can be built with separate tinfo lib. In that case configure check fails with

+ cat config.log
+ LDFLAGS=-lcurses
+ check_curses
+ compile
+ cc -Werror -o /dev/null -x c - -lcurses
/usr/lib/gcc/x86_64-pc-linux-gnu/9.1.0/../../../../x86_64-pc-linux-gnu/bin/ld: /var/tmp/portage/sys-apps/pick-3.0.1/temp/ccbghdyi.o: undefined reference to symbol 'setupterm'
/usr/lib/gcc/x86_64-pc-linux-gnu/9.1.0/../../../../x86_64-pc-linux-gnu/bin/ld: /lib64/libtinfo.so.6: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
+ LDFLAGS=-lncursesw
+ check_curses
+ compile
+ cc -Werror -o /dev/null -x c - -lncursesw
/usr/lib/gcc/x86_64-pc-linux-gnu/9.1.0/../../../../x86_64-pc-linux-gnu/bin/ld: /var/tmp/portage/sys-apps/pick-3.0.1/temp/ccOVkf2c.o: undefined reference to symbol 'setupterm'
/usr/lib/gcc/x86_64-pc-linux-gnu/9.1.0/../../../../x86_64-pc-linux-gnu/bin/ld: /lib64/libtinfow.so.6: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
+ fatal 'curses library not found'
+ '[' 1 -gt 0 ']'
+ echo 'fatal: curses library not found'
fatal: curses library not found

Thanks-to: Helmut Jarausch jarausch@igpm.rwth-aachen.de Gentoo-bug: https://bugs.gentoo.org/685712 Signed-off-by: Lars Wendler polynomial-c@gentoo.org

[mptre]

Thanks! Committed in cb71507 with some minor tweaks.