[m-rev.] for review: Enable munmap support in Boehm GC by default.

[Peter Wang]

Julien, do you want me to wait until after the next ROTD before
committing this?

----

configure:
    Change default value of enable_gc_mmap and enable_gc_munmap to yes.

    Invert sense of --enable-gc-mmap and --enable-gc-munmap options in
    the help text.

NEWS:
    Announce change.

diff --git a/NEWS b/NEWS
index 940cbb52a..46715f98f 100644
--- a/NEWS
+++ b/NEWS
@@ -943,4 +943,8 @@ Changes to the Mercury implementation
 
 * We have upgraded the bundled Boehm GC to v8.0.6 and libatomic_ops to v7.6.12.
 
+* We have enabled support for unmapping of unused memory pages in Boehm GC
+  by default. If this should cause a problem on your platform, it can be
+  disabled by passing `--disable-gc-munmap` to configure.
+
 For news about earlier versions, see the HISTORY file.
diff --git a/configure.ac b/configure.ac
index 78d3f8ed6..9109a9ba7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -708,14 +708,14 @@ AC_SUBST(ENABLE_BOEHM_LARGE_CONFIG)
 #
 
 AC_ARG_ENABLE(gc-mmap,
-    AS_HELP_STRING([--enable-gc-mmap],
-                   [use mmap instead of sbrk when using Boehm GC]),
-    [enable_gc_mmap="$enableval"],[enable_gc_mmap=no])
+    AS_HELP_STRING([--disable-gc-mmap],
+                   [use sbrk instead of mmap when using Boehm GC]),
+    [enable_gc_mmap="$enableval"],[enable_gc_mmap=yes])
 
 AC_ARG_ENABLE(gc-munmap,
-    AS_HELP_STRING([--enable-gc-munmap],
-                   [enable unmapping of unused pages when using Boehm GC]),
-    [enable_gc_munmap="$enableval"],[enable_gc_munmap=no])
+    AS_HELP_STRING([--disable-gc-munmap],
+                   [disable unmapping of unused pages when using Boehm GC]),
+    [enable_gc_munmap="$enableval"],[enable_gc_munmap=yes])
 
 AC_MSG_CHECKING(whether to enable unmapping of unused pages when using Boehm GC)
 case "$enable_gc_munmap" in
-- 
2.31.0