Changed every 'fpic' variant to 'pic' (#4969)

* Changed every 'fpic' variant to 'pic'. fixes #2463

Every variant that activates compilation of position independent code
has been changed to 'pic'. Hardcoded compiler flags in packages have
been substituted with `self.compiler.pic_flag`.

* Changed literal uses of '-fpic' to 'self.compiler.pic_flag'

var/spack/repos/builtin/packages/adios/package.py

@@ -103,13 +103,12 @@ def configure_args(self):
         spec = self.spec
         self.validate(spec)
 
-        extra_args = []
-
-        # required, otherwise building its python bindings on ADIOS will fail
-        extra_args.append("CFLAGS=-fPIC")
-
-        # always build external MXML, even in ADIOS 1.10.0+
-        extra_args.append('--with-mxml=%s' % spec['mxml'].prefix)
+        extra_args = [
+            # required, otherwise building its python bindings will fail
+            'CFLAGS={0}'.format(self.compiler.pic_flag),
+            # always build external MXML, even in ADIOS 1.10.0+
+            '--with-mxml={0}'.format(spec['mxml'].prefix)
+        ]
 
         if '+shared' in spec:
             extra_args.append('--enable-shared')