lang/python27: Fix build regression

r462630 added OpenSSL include/library paths to CFLAGS/LIBS which caused
the _elementtree and pyexpat modules to fail to build. Adding paths to CFLAGS
is known to cause issues [1].

Accordingly:

• Switch to using CPPFLAGS for OpenSSL include/library paths (like NLS).
• Move the comment describing why this is necessary to the top of the port with reasonable warning about what can happen and why.

[1] https://svnweb.freebsd.org/ports?view=revision&revision=326729

PR: 226135, 222795)
Reported by: many
Reviewed by: sunpoet (python)
Approved by: koobs (python)