cpan.bbclass: Fix CCFLAGS.

cpan.bbclass: Fix CCFLAGS.

We should not just replace CCFLAGS with CFLAGS while compiling, because
this may cause run-time errors with perl's DynaLoader.pm.

Tested on qemux86 with new libnet-libidn-perl bb recipe:

root@qemux86:~# perl -e "use Net::LibIDN"
Not a CODE reference at /usr/lib/perl/5.14.2//DynaLoader.pm line 213.
END failed--call queue aborted at /usr/lib/perl/vendor_perl/5.14.2//Net/LibIDN.pm line 213.
Compilation failed in require at -e line 1.
BEGIN failed--compilation aborted at -e line 1.

(From OE-Core rev: 855211ae7a224f96663862d4a0e58a90d945dd48)

Signed-off-by: Xin Ouyang <Xin.Ouyang@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>