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)

lang/python27/Makefile

@@ -14,12 +14,21 @@ COMMENT=	Interpreted object-oriented programming language
 
 LICENSE=	PSFL
 
+# lang/python* ports use CPPFLAGS over CFLAGS due to multiple complex
+# *FLAGS and include ordering bugs/issues in the Python build, that cause
+# modules such as elementtree and pyexpat to fail to build, among other
+# difficult to isolate issues. Do not add to or modify global variables
+# unless the implications and pitfalls are completely understood.
+# See: https://bugs.python.org/issue6299#msg210189
+
 USES=		cpe ncurses pathfix pkgconfig readline:port shebangfix ssl tar:xz
 PATHFIX_MAKEFILEIN=	Makefile.pre.in
 USE_LDCONFIG=	yes
-CFLAGS+=	-I${OPENSSLINC}
 GNU_CONFIGURE=	yes
-LIBS+=		-L${OPENSSLLIB}
+
+CPPFLAGS+=      -I${OPENSSLINC}
+LDFLAGS+=	-L${OPENSSLLIB}
+
 python_CMD=	${PREFIX}/bin/python${PYTHON_PORTVERSION:R}
 SHEBANG_FILES=	Lib/lib2to3/pgen2/*.py Lib/lib2to3/tests/*.py Lib/lib2to3/tests/data/*.py \
 		Lib/idlelib/*.py Lib/encodings/*.py Lib/test/*.py Lib/UserString.py \
@@ -60,9 +69,6 @@ IPV6_CONFIGURE_ENABLE=	ipv6
 LIBFFI_CONFIGURE_ON=	--with-system-ffi
 LIBFFI_LIB_DEPENDS=	libffi.so:devel/libffi
 
-# Use CPPFLAGS over CFLAGS due to -I ordering, causing elementtree and pyexpat
-# to break in Python 2.7, or preprocessor complaints in Python >= 3.3
-# Upstream Issue: https://bugs.python.org/issue6299
 NLS_USES=		gettext
 NLS_CPPFLAGS=		-I${LOCALBASE}/include
 NLS_LIBS=		-L${LOCALBASE}/lib -lintl