UPDATE: this is slightly more mature now. Check out http://code.google.com/p/android-cruft/wiki/SL4AC
Today I played around with the Android NDK, a C compiler for Android. The Android folks make it very clear that it's only intended for making ordinary Java-based Android apps faster, by running a few selected functions down at the bare metal.
So it only comes with a few basic libraries which don't support things like reading the sensors and taking photos.
Enter SL4A, Scripting Languages for Android (formerly "ASE", the Android Scripting Environment). SL4A is a neat Android app that makes it very easy to write simple Android apps in many popular scripting languages like Python, Perl, Ruby and TCL. SL4A exposes a useful subset of the Android API in a clever way: by setting up a JSON RPC server. That way, each language only needs to implement a thin RPC client layer to access the whole SL4A API.
So today I managed to get the excellent Jansson C-language JSON library to compile with the NDK, making it possible for native C apps to access the Android API through SL4A, just as its normally supported languages can.
It works just fine as either a native Android binary or from a host machine, so you could also use it to control your phone from a C program running on a host computer.
Here's the ndk-to-sl4a.c source code, public domain. See the comments in the header for instructions on how to compile and run it.
If you're the trusting sort, you can also download a binary for froyo or for (ubuntu/lucid) linux. But you should still go through the comments in the source file to see how to set up SL4A first.