[PATCH] handle libc implemntations which do not provide `error.h`

[Saleem Abdulrasool <abdulras@google.com>]

Introduce a configure time check for the presence of `error.h`.  In the
case that `error.h` is not available, we can fall back to `err.h`.
Although `err.h` is not a C standard header (it is a BSD extension),
many libc implementations provide.  If there are targets which do not
provide an implementation of `err.h`, it would be possible to further
extend the implementation to be more portable.

This resolves PR21008.

Signed-off-by: Saleem Abdulrasool <abdulras@google.com>
---
 configure.ac |  3 +++
 lib/system.h | 23 ++++++++++++++++++++++-
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/configure.ac b/configure.ac
index 7caff2c5..177bb1a2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -431,6 +431,9 @@ AC_CHECK_DECLS([reallocarray],[],[],
 
 AC_CHECK_FUNCS([process_vm_readv])
 
+AC_CHECK_HEADERS([error.h])
+AC_CHECK_HEADERS([err.h])
+
 old_CFLAGS="$CFLAGS"
 CFLAGS="$CFLAGS -D_GNU_SOURCE"
 AC_FUNC_STRERROR_R()
diff --git a/lib/system.h b/lib/system.h
index 58d9deee..8adb5848 100644
--- a/lib/system.h
+++ b/lib/system.h
@@ -29,8 +29,9 @@
 #ifndef LIB_SYSTEM_H
 #define LIB_SYSTEM_H	1
 
+#include <config.h>
+
 #include <errno.h>
-#include <error.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <sys/param.h>
@@ -40,6 +41,26 @@
 #include <string.h>
 #include <stdlib.h>
 
+#if defined(HAVE_ERROR_H)
+#include <error.h>
+#elif defined(HAVE_ERR_H)
+static int error_message_count = 0;
+
+static inline void error(int status, int errnum, const char *format, ...) {
+  va_list argp;
+
+  va_start(argp, format);
+  verr(status, format, argp);
+  va_end(argp);
+
+  if (status)
+    exit(status);
+  ++error_message_count;
+}
+#else
+#error "err.h or error.h must be available"
+#endif
+
 #if __BYTE_ORDER == __LITTLE_ENDIAN
 # define LE32(n)	(n)
 # define LE64(n)	(n)
-- 
2.33.0.rc2.250.ged5fa647cd-goog