Jamfile: Optimize build output size

The goal here is to keep builds reasonably fast for development.
Some time hopefully we'll have the ability to specify whether to build
a debug or release build. But for now, we're stuck with trying to
optimize both build time and build size simultaneously.

Coolcoder613eb proposed four build flags to add. I tested them for
their effect on build time and size.

Jamfile

@@ -82,7 +82,9 @@ SubDirHdrs /system/lib/python$(python_version)/vendor-packages/pybind11/include/
 SubDirHdrs /system/develop/headers/python$(python_version)/ ;
 
 # Additional C++ flags to use when compiling
-SubDirC++Flags -std=c++14 -pipe -fPIC ;
+# TODO: only run -Os for release builds. Development builds should prioritize
+# build time
+SubDirC++Flags -std=c++14 -pipe -fPIC -Os -fvisibility=hidden ;
 
 # Flags to use when linking
 LINKLIBS = -lbe ;