package/exiv2: fix compiler flags detection

package/exiv2: fix compiler flags detection

Add a patch, which has been submitted upstream, to properly detect the
availability of compiler flags before using them.

This will fix the build of exiv2 on CPU architectures for which gcc
doesn't implement -fcf-protection.

Fixes:

  http://autobuild.buildroot.net/results/ef6a6fa2309e66a7a36fface626ae0da1a7ec633/
  (and many similar build failures on other architectures)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[yann.morin.1998@free.fr:
  - fix indentation s/TAB/spaces/
  - fix CRLF dropped by patchwork
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>