Update to 5.1.8

• While I'm here [1]:
  • Use CONFLICTS_INSTALL instead of CONFLICTS
  • Use USES=localbase

Changes: https://pecl.php.net/package-changelog.php?package=APCu
PR: 216968
Submitted by: John Chen <johnpupu@gmail.com> (maintainer)
Differential Revision: https://reviews.freebsd.org/D9333 [1]