Config,Darwin: Allow for configuring Darwin to use embedded runpath.

Recent Darwin versions place contraints on the use of run paths
specified in environment variables.  This breaks some assumptions
in the GCC build.

This change allows the user to configure a Darwin build to use
'@rpath/libraryname.dylib' in library names and then to add an
embedded runpath to executables (and libraries with dependents).

The embedded runpath is added by default unless the user adds
'-nodefaultrpaths' to the link line.

For an installed compiler, it means that any executable built with
that compiler will reference the runtimes installed with the
compiler (equivalent to hard-coding the library path into the name
of the library).

During build-time configurations  any "-B" entries will be added to
the runpath thus the newly-built libraries will be found by exes.

Since the install name is set in libtool, that decision needs to be
available here (but might also cause dependent ones in Makefiles,
so we need to export a conditional).

This facility is not available for Darwin 8 or earlier, however the
existing environment variable runpath does work there.

We default this on for systems where the external DYLD_LIBRARY_PATH
does not work and off for Darwin 8 or earlier.  For systems that can
use either method, if the value is unset, we use the default (which
is currently DYLD_LIBRARY_PATH).

configure

@@ -682,6 +682,8 @@ PIC_FLAG
 WARN_FLAGS
 EXTRA_FLAGS
 BACKTRACE_FILE
+ENABLE_DARWIN_AT_RPATH_FALSE
+ENABLE_DARWIN_AT_RPATH_TRUE
 OTOOL64
 OTOOL
 LIPO
@@ -11393,7 +11395,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 11396 "configure"
+#line 11398 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -11499,7 +11501,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 11502 "configure"
+#line 11504 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -11738,6 +11740,15 @@ CC="$lt_save_CC"
 
 
 
+ if test x$enable_darwin_at_rpath = xyes; then
+  ENABLE_DARWIN_AT_RPATH_TRUE=
+  ENABLE_DARWIN_AT_RPATH_FALSE='#'
+else
+  ENABLE_DARWIN_AT_RPATH_TRUE='#'
+  ENABLE_DARWIN_AT_RPATH_FALSE=
+fi
+
+
 # Check whether --enable-largefile was given.
 if test "${enable_largefile+set}" = set; then :
   enableval=$enable_largefile;
@@ -13662,6 +13673,10 @@ if test -z "${HAVE_DWZ_TRUE}" && test -z "${HAVE_DWZ_FALSE}"; then
   as_fn_error $? "conditional \"HAVE_DWZ\" was never defined.
 Usually this means the macro was only invoked conditionally." "$LINENO" 5
 fi
+if test -z "${ENABLE_DARWIN_AT_RPATH_TRUE}" && test -z "${ENABLE_DARWIN_AT_RPATH_FALSE}"; then
+  as_fn_error $? "conditional \"ENABLE_DARWIN_AT_RPATH\" was never defined.
+Usually this means the macro was only invoked conditionally." "$LINENO" 5
+fi
 if test -z "${HAVE_ELF_TRUE}" && test -z "${HAVE_ELF_FALSE}"; then
   as_fn_error $? "conditional \"HAVE_ELF\" was never defined.
 Usually this means the macro was only invoked conditionally." "$LINENO" 5

configure.ac

@@ -90,6 +90,8 @@ AM_CONDITIONAL(HAVE_DWZ, test "$DWZ" != "")
 LT_INIT
 AM_PROG_LIBTOOL
 
+AM_CONDITIONAL([ENABLE_DARWIN_AT_RPATH], [test x$enable_darwin_at_rpath = xyes])
+
 AC_SYS_LARGEFILE
 
 backtrace_supported=yes