sagemathgh-36599: sage-env: identify the version of command-line tools (OS X) and set LDFLAGS accordingly
    
Identify the version of command-line tools in use (OS X), and use the
version to set LDFLAGS.

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In sage-env, save the version of command-line tools being used in
XCLT_VERSION. If it's too recent, we set LDFLAGS to solve some problems:
- Fixes sagemath#36337 (doctest failures)
- Fixes sagemath#36342 (scipy building,
replacing sagemath#36523).

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URL: sagemath#36599
Reported by: John H. Palmieri
Reviewer(s): Matthias Köppe

Author: Release Manager

src/bin/sage-env

@@ -374,7 +374,14 @@ if [ -n "$PYTHONHOME" ]; then
 fi
 
 if [ -n "$SAGE_LOCAL" ]; then
-    LDFLAGS="-L$SAGE_LOCAL/lib -Wl,-rpath,$SAGE_LOCAL/lib $LDFLAGS"
+    # On OS X, test whether "ld-classic" is present in the installed
+    # version of the command-line tools. If so, we add "-ld_classic"
+    # to LD_FLAGS. See #36599.
+    if [ "$UNAME" = "Darwin" ] && [ -x "$(xcode-select -p)/usr/bin/ld-classic" ] ; then
+        LDFLAGS="-L$SAGE_LOCAL/lib -Wl,-ld_classic,-rpath,$SAGE_LOCAL/lib $LDFLAGS"
+    else
+        LDFLAGS="-L$SAGE_LOCAL/lib -Wl,-rpath,$SAGE_LOCAL/lib $LDFLAGS"
+    fi
     if [ "$UNAME" = "Linux" ]; then
         LDFLAGS="-Wl,-rpath-link,$SAGE_LOCAL/lib $LDFLAGS"
     fi