Add variant to build shared Perl lib (#4416)

* Add variant to build shared Perl lib

Add a variant that enables Perl's "useshrplib" feature, which builds a
shared perl library.

This addresses problems like so:

```
/usr/bin/ld: /blah/blah/spack/opt/spack/linux-centos7-x86_64/gcc-4.8.5/perl-5.24.1-y43dp3p5w66v7qh5xkwgufxohyuodyew/lib/5.24.1/x86_64-linux/CORE/libperl.a(op.o): relocation R_X86_64_32S against `PL_opargs' can not be used when making a shared object; recompile with -fPIC
/blah/blah/spack/opt/spack/linux-centos7-x86_64/gcc-4.8.5/perl-5.24.1-y43dp3p5w66v7qh5xkwgufxohyuodyew/lib/5.24.1/x86_64-linux/CORE/libperl.a: could not read symbols: Bad value
```

It should also address the Intel compiler issue discussed in #3081
while respecting Perl's configuration machinery.

* Rename shared variant and default to True

* Use correct variant to add configure arg

* Restore bits that set ccflags for intel compilers

After some experimentation we've established that setting
the flag to build a shared perl library is tightly tied to
the use of -fPIC.

This commit restores the code that sets ccflags for
intel compilers.

* Flake8 cleanup

1 files changed, 9 insertions, 2 deletions

diff --git a/var/spack/repos/builtin/packages/perl/package.py b/var/spack/repos/builtin/packages/perl/package.py
index ef1c11289f..95b2ae8726 100644
--- a/var/spack/repos/builtin/packages/perl/package.py
+++ b/var/spack/repos/builtin/packages/perl/package.py
@@ -69,6 +69,9 @@ class Perl(Package):  # Perl doesn't use Autotools, it should subclass Package
     variant('cpanm', default=True,
             description='Optionally install cpanm with the core packages.')
 
+    variant('shared', default=True,
+            description='Build a shared libperl.so library')
+
     resource(
         name="cpanm",
         url="http://search.cpan.org/CPAN/authors/id/M/MI/MIYAGAWA/App-cpanminus-1.7042.tar.gz",
@@ -87,14 +90,18 @@ class Perl(Package):  # Perl doesn't use Autotools, it should subclass Package
             '-des',
             '-Dprefix={0}'.format(prefix),
             '-Dlocincpth=' + self.spec['gdbm'].prefix.include,
-            '-Dloclibpth=' + self.spec['gdbm'].prefix.lib
+            '-Dloclibpth=' + self.spec['gdbm'].prefix.lib,
         ]
 
         # Discussion of -fPIC for Intel at:
-        # https://github.com/LLNL/spack/pull/3081
+        # https://github.com/LLNL/spack/pull/3081 and
+        # https://github.com/LLNL/spack/pull/4416
         if spec.satisfies('%intel'):
             config_args.append('-Accflags={0}'.format(self.compiler.pic_flag))
 
+        if '+shared' in spec:
+            config_args.append('-Duseshrplib')
+
         return config_args
 
     def configure(self, spec, prefix):