Explicitly search for pkg-config in configure script

platform/unix/configure.ac

@@ -13,6 +13,7 @@ AC_PROG_CXX
 AC_PROG_SED
 AC_PROG_MKDIR_P
 AC_PROG_OBJCXX
+PKG_PROG_PKG_CONFIG
 AC_C_BIGENDIAN
 AC_LANG([C++])