stdbuf: support compilers other than __GNUC__

__SUNPRO_C >= 0x590 /*12.0*/ supports __attribute((constructor))
required by stdbuf, so use a more direct check for this.

Note ensure that --libexecdir is set to the appropriate
install location for libstdbuf.so so that stdbuf works
when installed on the system like it does when running
tests in the build directory.

* configure.ac (stdbuf_supported): Use a test prog to determine support.
* src/libstdbuf.c (stdbuf): Define appropriately for non GCC compilers,
and provide early feedback (compilation warning) if trying to compile
libstdbuf without the necessary support.
* src/stdbuf.c (set_LD_PRELOAD): Add a note on having stdbuf
look for libstdbuf.so in the default lib search path.
* cfg.mk (sc_prohibit-gl-attributes): Adjust so we can exclude
libstdbuf.so from prohibiting '__attribute', since we want
this form to avoid silently eliding this required attribute on non GCC.

Reported and tested by Rich Burridge.

2 files changed, 13 insertions, 2 deletions

diff --git a/src/libstdbuf.c b/src/libstdbuf.c
index 88321e9e4..1281b9de7 100644
--- a/src/libstdbuf.c
+++ b/src/libstdbuf.c
@@ -127,7 +127,8 @@ apply_mode (FILE *stream, const char *mode)
     }
 }
 
-__attribute__ ((constructor)) static void
+/* Use __attribute to avoid elision of __attribute__ on SUNPRO_C etc.  */
+static void __attribute ((constructor))
 stdbuf (void)
 {
   char *e_mode = getenv ("_STDBUF_E");
diff --git a/src/stdbuf.c b/src/stdbuf.c
index 270881bf5..c648fc5d0 100644
--- a/src/stdbuf.c
+++ b/src/stdbuf.c
@@ -195,7 +195,17 @@ set_LD_PRELOAD (void)
      However we want the lookup done for the exec'd command not stdbuf.
 
      Since we don't link against libstdbuf.so add it to PKGLIBEXECDIR
-     rather than to LIBDIR.  */
+     rather than to LIBDIR.
+
+     Note we could add "" as the penultimate item in the following list
+     to enable searching for libstdbuf.so in the default system lib paths.
+     However that would not indicate an error if libstdbuf.so was not found.
+     Also while this could support auto selecting the right arch in a multilib
+     environment, what we really want is to auto select based on the arch of the
+     command being run, rather than that of stdbuf itself.  This is currently
+     not supported due to the unusual need for controlling the stdio buffering
+     of programs that are a different architecture to the default on the
+     system (and that of stdbuf itself).  */
   char const *const search_path[] = {
     program_path,
     PKGLIBEXECDIR,