Bug 1360300 -- Support enabling PIE when using clang. r=froydnj

Also remove bits of a comment, now that we support only macOS 10.9+, most of the
comment isn't relevant.

While PIE is enabled by default on macOS, this isn't true of clang on Linux.
--enable-pie can now be used with clang on Linux.

r=froydnj

MozReview-Commit-ID: rc6zJiWzLo

--HG--
extra : rebase_source : 3745175e106ea8c6be9271d8135d43ba359434c7

build/autoconf/compiler-opts.m4

@@ -251,11 +251,7 @@ if test "$GNU_CC" -a "$GCC_USE_GNU_LD" -a -z "$DEVELOPER_OPTIONS"; then
 fi
 
 # bionic in Android < 4.1 doesn't support PIE
-# On OSX, the linker defaults to building PIE programs when targetting OSX 10.7+,
-# but not when targetting OSX < 10.7. OSX < 10.7 doesn't support running PIE
-# programs, so as long as support for OSX 10.6 is kept, we can't build PIE.
-# Even after dropping 10.6 support, MOZ_PIE would not be useful since it's the
-# default (and clang says the -pie option is not used).
+# On OSX, the linker defaults to building PIE programs when targeting OSX 10.7.
 # On other Unix systems, some file managers (Nautilus) can't start PIE programs
 if test -n "$gonkdir" && test "$ANDROID_VERSION" -ge 16; then
     MOZ_PIE=1
@@ -268,7 +264,7 @@ MOZ_ARG_ENABLE_BOOL(pie,
     MOZ_PIE=1,
     MOZ_PIE= )
 
-if test "$GNU_CC" -a -n "$MOZ_PIE"; then
+if test "$GNU_CC$CLANG_CC" -a -n "$MOZ_PIE"; then
     AC_MSG_CHECKING([for PIE support])
     _SAVE_LDFLAGS=$LDFLAGS
     LDFLAGS="$LDFLAGS -pie"