fix compilation error in C++ mode on OpenBSD

[Bruno Haible]

On OpenBSD, a testdir created with --with-c++-tests produces this compilation 
error:

depbase=`echo test-spawn-c++.o | sed 's|[^/]*$|.deps/&|;s|\.o$||'`; g++ 
-DHAVE_CONFIG_H -DEXEEXT=\"\" -I. -I../../gltests -I..   
-DGNULIB_STRICT_CHECKING=1  -DIN_GNULIB_TESTS=1  -I. -I../../gltests  -I.. 
-I../../gltests/..  -I../gllib -I../../gltests/../gllib -I/home/bruno/include 
-Wall   -MT test-spawn-c++.o -MD -MP -MF $depbase.Tpo -c -o test-spawn-c++.o 
../../gltests/test-spawn-c++.cc && mv -f $depbase.Tpo $depbase.Po
In file included from ../gllib/sys/select.h:110,
                 from /usr/include/sys/time.h:38,
                 from ../gllib/sys/time.h:39,
                 from /usr/include/sys/siginfo.h:130,
                 from /usr/include/sys/signal.h:107,
                 from /usr/include/spawn.h:34,
                 from ../gllib/spawn.h:28,
                 from ../../gltests/test-spawn-c++.cc:22:
../gllib/signal.h: In member function 'gnulib::_gl_signal_wrapper::operator 
void (* (*)(int, void (*)(int)))(int)() const':
../gllib/signal.h:799: error: '::signal' has not been declared
...
Build for test-spawn-c++ aborted

The reason is that
  - On OpenBSD, signal() is declared near the end of <sys/signal.h>, not
    in <signal.h>.
  - <signal.h> includes <sys/signal.h>, but in this case, <spawn.h>
    includes <sys/signal.h> directly, which has the effect that when <signal.h>
    includes <sys/signal.h>, this include does nothing.
  - Thus gnulib's <signal.h> use of ::signal occurs before the function gets
    declared.

This patch fixes it.


2019-12-08  Bruno Haible  <br...@clisp.org>

        Fix compilation error in C++ mode on OpenBSD.
        * lib/signal.in.h (signal): Declare on OpenBSD.

diff --git a/lib/signal.in.h b/lib/signal.in.h
index b4e432d..5b36c25 100644
--- a/lib/signal.in.h
+++ b/lib/signal.in.h
@@ -322,6 +322,12 @@ _GL_FUNCDECL_RPL (signal, 
_gl_function_taking_int_returning_void_t,
 _GL_CXXALIAS_RPL (signal, _gl_function_taking_int_returning_void_t,
                   (int sig, _gl_function_taking_int_returning_void_t func));
 # else
+/* On OpenBSD, the declaration of 'signal' may not be present at this point,
+   because it occurs in <sys/signal.h>, not <signal.h> directly.  */
+#  if defined __OpenBSD__
+_GL_FUNCDECL_SYS (signal, _gl_function_taking_int_returning_void_t,
+                  (int sig, _gl_function_taking_int_returning_void_t func));
+#  endif
 _GL_CXXALIAS_SYS (signal, _gl_function_taking_int_returning_void_t,
                   (int sig, _gl_function_taking_int_returning_void_t func));
 # endif