Update documentation w/regard to C11 compatibility discovery

[skip ci]

Author: saxbophone

CONTRIBUTING.md

@@ -25,6 +25,7 @@ In addition, please make sure:
 
 - You commit files with Unix Line-endings (`\n` `<LF>` `0x0a`)
 - Each text file committed has a trailing newline at the end
+- Your C code is compliant to the ISO C99 and C11 standards
 - C source code is indented with 4 spaces per indentation level (no tabs)
 - Public functions are prototyped in the correct C Header file, all private declarations are declared `static`
 - Every public function (and ideally private too) has an accompanying explanatory comment

README.md

@@ -42,15 +42,15 @@ Libsaxbospiral can be built without installing for test purposes and for general
 
 You will need:
 
-- A compiler that can compile ISO C99 code
+- A compiler that can compile ISO C99 or C11 code
 - [Cmake](https://cmake.org/) - v3.0 or newer
 - [libpng](http://www.libpng.org/pub/png/libpng.html) - (this often comes preinstalled with many modern unix-like systems)
 
 > ### Note:
 
 > These commands are for unix-like systems, without an IDE or other build system besides CMake. If building for a different system, or within an IDE or other environment, consult your IDE/System documentation on how to build CMake projects.
 
-> Additionally, it is of worth noting that this library has only been thoroughly tested and developed on **Ubuntu GNU/Linux** with **GCC v5.4.0** and **Clang 3.8.0**. Although every effort has been made to make it as cross-platform as possible (including quite strict **ISO C 99** compliance), **Your Mileage May Vary**. Bug Reports and Patches for problems running on other systems, particularly **Microsoft Windows** and **Mac OSX** are most welcome.
+> Additionally, it is of worth noting that this library has only been thoroughly tested and developed on **Ubuntu GNU/Linux** with **GCC v5.4.0** and **Clang 3.8.0**. Although every effort has been made to make it as cross-platform as possible (including quite strict **ISO C99** and **ISO C11** compliance), **Your Mileage May Vary**. Bug Reports and Patches for problems running on other systems, particularly **Microsoft Windows** and **Mac OSX** are most welcome.
 
 ### Recommended Library Build
 
@@ -61,6 +61,14 @@ cmake -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=ON .
 make
 ```
 
+The above builds in C99 mode by default. The standard to use is controlled by the `LIBSAXBOSPIRAL_C_STANDARD` environment variable.
+
+You can build in C11 mode if you want with the following:
+
+```sh
+LIBSAXBOSPIRAL_C_STANDARD=11 cmake -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=ON .
+```
+
 > ### Note:
 
 > Building as a shared library is recommended as then binaries compiled from [sxbp](https://github.com/saxbophone/sxbp) or your own programs that are linked against the shared version can immediately use any installed upgraded versions of libsaxbospiral with compatible ABIs without needing re-compiling.