Replaced PKG_CHECK_MODULES with AC_CHECKs in configure.ac.

Review: https://reviews.apache.org/r/25030
Replaced PKG_CHECK_MODULES with AC_CHECKs in libprocess configure.ac.

Review: https://reviews.apache.org/r/25031
Switch to clang by default

Also: Allow release builds when using clang as the system compiler.

The primary motivation for this is MESOS-1734. Clang compiles faster.