manual: complete example in error message documentation

The manual gives "an example showing how to handle failure to open a
file correctly."  The example function, open_sesame, uses the
newly-introduced strerror function and errno and
program_invocation_short_name variables.  It fails to specify GNU
extensions, however, so attempts to use it in the following way:

    int main (void) {open_sesame ("badname");}

fail during compilation with "error: ‘program_invocation_short_name’
undeclared", indicating the example is incomplete.  The presence of
"#include"s suggest everything neccesary for the function to work should
be present.  For completeness, the example is lacking the following line:

    #define _GNU_SOURCE

as the declarations of program_invocation_*name in errno.h are wrapped
in an "#ifdef __USE_GNU" conditional.

The documentation of the variables is also expanded, adding that their
definition lies in errno.h and noting specifically they are GNU
extensions.

1 files changed, 8 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index cda9af2..c7873da 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2015-03-06  Rical Jasan  <ricaljasan@pacific.net>
+
+	* manual/errno.texi (Error Messages): Complete example function
+	by adding missing #define.
+	(program_invocation_name): Add statement indicating GNU
+	extension and reference which header file declares the variable.
+	(program_invocation_short_name): Likewise.
+
 2015-03-06  Mike Frysinger  <vapier@gentoo.org>
 
 	* manual/errno.texi (Error Messages): Delete strerror ISO C89