lxdialog: code cleanups

Fixed some lxdialog compiler warnings and removed an unused
variable. The unused "y" variable cannot be removed because
the ncurses standard interface getyx() requires it.
lxdialog: update headers and C flags for modern systems

The Makefile in scripts/lxdialog used -fomit-frame-pointer but
there is no reason to use it. Added flags for C99 and pedantic
warnings. Also added strings.h to dialog.h since some of the
string functions are supposed to use it. Without strings.h there
were many implicit function declaration warnings.