poco: Fix the build on Linux
poco: Fix the build on Linux

Closes Homebrew/linuxbrew#98

Linuxbrew/homebrew-core,maxim-belkin/homebrew-core,Linuxbrew/homebrew-core,rwhogg/homebrew-core,forevergenin/homebrew-core,LinuxbrewTestBot/homebrew-core,forevergenin/homebrew-core,rwhogg/homebrew-core,maxim-belkin/homebrew-core,LinuxbrewTestBot/homebrew-core