Update build.md

[tortfeaser]

Added pkgconfig to FreeBSD dependencies. Without pkgconfg cmake will result in error

CMake Error at /usr/local/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find PkgConfig (missing: PKG_CONFIG_EXECUTABLE)
Call Stack (most recent call first):
  /usr/local/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:600 (_FPHSA_FAILURE_MESSAGE)
  /usr/local/share/cmake/Modules/FindPkgConfig.cmake:99 (find_package_handle_standard_args)
  CMakeLists.txt:167 (find_package)

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---

Pull Request Checklist

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[badaix]

Makes sense, thanks!