Build fixes for libepoll-shim

• correct compiler flags when using GCC; clears BROKEN on PPC64
• correct spelling of INCSDIR; fixes build when PREFIX != /usr/local [1]

PR: 219576 [1]
Submitted by: John Hein [1]
Reported by: Mikhail T. [1]
Approved by: swills (mentor)
Differential Revision: https://reviews.freebsd.org/D11551