Improve build time when updating Boost flags

[varunagrawal]

I came across an interesting article that gives recommendations on how to set preprocessor flags such that it improves build time.

The key idea is to get rid of add_definitions calls in CMake since it passes the preprocessor flag to all cpp files, and instead use a definition within the config.h.in file. This would then only require re-compiling files that include gtsam/config.h and not the complete project in the event the flags were updated.

Since we already have the config.h.in, this was an easy update.

I recommend reading the article, since it also discusses the benefits of using #cmakedefine01 over #cmakedefine.

[dellaert]

LGTM but maybe get review from @jlblancoc ? ROS interactions?

[jlblancoc]

Hi,
I totally agree with this change, @varunagrawal !!

My only comment is: wouldn't the #ifdef XX need to be #if XXX if using #cmakedefine01 ? (didn't test myself)

[varunagrawal]

@jlblancoc great catch! I'll update it.

[jlblancoc]

Now, it LGTM!