Fallback to stub-termcap.c on all hosts

Currently building gdb is impossible without an installed termcap or
curses library.  But, GDB already has a very minimal termcap in the
tree to handle this situation for Windows -- gdb/stub-termcap.c.  This
patch makes that the fallback for all hosts.

Testing this on GNU/Linux (by simply hacking away the termcap/curses
detection in gdb/configure.ac), we trip on:

 ../readline/libreadline.a(terminal.o): In function `_rl_init_terminal_io':
 /home/pedro/gdb/mygit/src/readline/terminal.c:527: undefined reference to `PC'
 /home/pedro/gdb/mygit/src/readline/terminal.c:528: undefined reference to `BC'
 /home/pedro/gdb/mygit/src/readline/terminal.c:529: undefined reference to `UP'
 /home/pedro/gdb/mygit/src/readline/terminal.c:538: undefined reference to `PC'
 /home/pedro/gdb/mygit/src/readline/terminal.c:539: undefined reference to `BC'
 /home/pedro/gdb/mygit/src/readline/terminal.c:540: undefined reference to `UP'

These are globals that are normally defined by termcap (or ncurses'
termcap emulation).

Now, we could just define replacements in stub-termcap.c, but
readline/terminal.c (at least the copy in our tree) has this:

 #if !defined (__linux__) && !defined (NCURSES_VERSION)
 #  if defined (__EMX__) || defined (NEED_EXTERN_PC)
 extern
 #  endif /* __EMX__ || NEED_EXTERN_PC */
 char PC, *BC, *UP;
 #endif /* !__linux__ && !NCURSES_VERSION */

which can result in readline defining the globals too.  That will
usually work out in C, given that "-fcommon" is usually the default
for C compilers, but that won't work for C++, or C with -fno-common
(link fails with "multiple definition" errors)...

Mirroring those #ifdef conditions in the stub termcap screams
"brittle" to me -- I can see them changing in latter readline
versions.

Work around that by simply using __attribute__((weak)).
Windows/PE/COFF's do support weak, but not on gcc 3.4 based toolchains
(4.8.x does work).  Given the file never needed the variables while it
was Windows-only, just continue not defining them there.  All other
supported hosts should support this.

gdb/ChangeLog:
2015-04-06  Pedro Alves  <palves@redhat.com>
	    Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* configure.ac: Remove the mingw32-specific stub-termcap.o
	fallback, and instead fallback to the stub termcap on all hosts.
	* configure: Regenerate.
	* stub-termcap.c [!__MINGW32__] (PC, BC, UP): Define as weak
	symbols.

1 files changed, 1 insertions, 6 deletions

diff --git a/gdb/configure.ac b/gdb/configure.ac
index ace033e..c703e35 100644
--- a/gdb/configure.ac
+++ b/gdb/configure.ac
@@ -617,18 +617,13 @@ case $host_os in
   go32* | *djgpp*)
     ac_cv_search_tgetent="none required"
     ;;
-  *mingw32*)
-    if test x"$curses_found" != xyes; then
-      ac_cv_search_tgetent="none required"
-      CONFIG_OBS="$CONFIG_OBS stub-termcap.o"
-    fi ;;
 esac
 
 # These are the libraries checked by Readline.
 AC_SEARCH_LIBS(tgetent, [termcap tinfo curses ncurses])
 
 if test "$ac_cv_search_tgetent" = no; then
-  AC_MSG_ERROR([no termcap library found])
+  CONFIG_OBS="$CONFIG_OBS stub-termcap.o"
 fi
 
 AC_ARG_WITH([system-readline],