perl: fix macOS build (#26290)
perl: fix macOS build (#26290)

* perl: fix macOS build

With both 5.34.0 and 5.32.1 the build fails on macos-bigsur-skylake
%clang@12.0.5 and %clang13.0.0 :
```
2 errors found in build log:
     579013    /private/var/folders/fy/x2xtwh1n7fn0_0q2kk29xkv9vvmbqb/T/s3j/spack-stage/spack-stage-perl-5.34.0-tpha2u52qfwaraidpzzbf6u4dbqg7dk5/spack-src/cpan/
               Math-BigInt-FastCalc/../../miniperl "-I../../lib" -MExtUtils::Command::MM -e 'cp_nonempty' -- FastCalc.bs ../../lib/auto/Math/BigInt/FastCalc/Fas
               tCalc.bs 644
     579014
     579015    	Everything is up to date. Type '/Applications/Xcode.app/Contents/Developer/usr/bin/make test' to run test suite.
     579016    DYLD_LIBRARY_PATH=/private/var/folders/fy/x2xtwh1n7fn0_0q2kk29xkv9vvmbqb/T/s3j/spack-stage/spack-stage-perl-5.34.0-tpha2u52qfwaraidpzzbf6u4dbqg7d
               k5/spack-src  ./perl -Ilib -I. installperl --destdir=
     579017    WARNING: You've never run 'make test' or some tests failed! (Installing anyway.)
     579018      /rnsdhpc/code/spack/opt/spack/apple-clang/perl/tpha2u5/bin/perl5.34.0
  >> 579019    install_name_tool: error: sh -c '/Applications/Xcode.app/Contents/Developer/usr/bin/xcodebuild -sdk /Applications/Xcode.app/Contents/Developer/Pl
               atforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -find install_name_tool 2> /dev/null' failed with exit code 256: (null) (errno=Invalid argument
               )
     579020    xcode-select: Failed to locate 'install_name_tool', requesting installation of command line developer tools.
     579021    Cannot update /rnsdhpc/code/spack/opt/spack/apple-clang/perl/tpha2u5/bin/perl5.34.0 dependency paths
  >> 579022    make: *** [install-all] Error 72
```
This is due to SYSTEM_VERSION_COMPAT being set.

* perl: conditionally set SYSTEM_VERSION_COMPAT based on CLT

The version of command line tools is the only difference between
@alalazo and my builds: his (v11) works only when SYSTEM_VERSION_COMPAT
is set to 1, and mine (v12.5 and v13) only work when it is unset.
LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack