Modify backslash/newline handling for POSIX.

We fixed Savannah 16670 but that broke previously-working makefiles
that relied on the GNU make behavior.  The POSIX behavior doesn't
seem to me to be better, and can be obtained using GNU make as well,
so put it back as the default behavior and require .POSIX to
get the POSIX behavior.
Add a new section to the manual discussing backslash/newline handling.
Update the test suite.

1 files changed, 8 insertions, 3 deletions

diff --git a/misc.c b/misc.c
index 55b0dd8..0ea56bb 100644
--- a/misc.c
+++ b/misc.c
@@ -112,12 +112,17 @@ collapse_continuations (char *line)
       /* Skip the newline.  */
       ++in;
 
-      /* If the newline is escaped, discard following whitespace leaving just
-	 one space.  POSIX requires that each backslash/newline/following
-	 whitespace sequence be reduced to a single space.  */
       if (backslash)
 	{
+          /* Backslash/newline handling:
+             In traditional GNU make all trailing whitespace, consecutive
+             backslash/newlines, and any leading whitespace on the next line
+             is reduced to a single space.
+             In POSIX, each backslash/newline and is replaced by a space.  */
 	  in = next_token (in);
+          if (! posix_pedantic)
+            while (out > line && isblank ((unsigned char)out[-1]))
+              --out;
 	  *out++ = ' ';
 	}
       else