* 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
23474be4b0