Fix tui compilation with Solaris libcurses: clear define (PR tui/21482)

Fix tui compilation with Solaris libcurses: clear define (PR tui/21482)

On both mainline and the 8.0 branch, gdb compilation fails on Solaris 10
with the native libcurses like this:

In file included from /vol/src/gnu/gdb/gdb-8.0-branch/local/gdb/gdb_curses.h:42:
0,
                 from /vol/src/gnu/gdb/gdb-8.0-branch/local/gdb/tui/tui-data.h:2
6,
                 from /vol/src/gnu/gdb/gdb-8.0-branch/local/gdb/tui/tui-disasm.c
:31:
/vol/src/gnu/gdb/gdb-8.0-branch/local/gdb/tui/tui-disasm.c: In function `CORE_A
DDR tui_disassemble(gdbarch*, tui_asm_line*, CORE_ADDR, int)':
/vol/src/gnu/gdb/gdb-8.0-branch/local/gdb/tui/tui-disasm.c:71:19: error: `class
 string_file' has no member named `wclear'; did you mean `clear'?
       gdb_dis_out.clear ();
                   ^
/vol/src/gnu/gdb/gdb-8.0-branch/local/gdb/tui/tui-disasm.c:78:19: error: `class
 string_file' has no member named `wclear'; did you mean `clear'?
       gdb_dis_out.clear ();
                   ^
make[2]: *** [Makefile:1927: tui-disasm.o] Error 1

It turned out this happens because <curses.h> has

#define clear()         wclear(stdscr)

This can be avoided by defining NOMACROS, which the patch below does.
ncurses potentially has a similar problem, which can be avoided by defining
NCURSES_NOMACROS.

	PR tui/21482
	* gdb_curses.h (NOMACROS): Define.
	(NCURSES_NOMACROS): Define.