CMakeLists.txt: Don't require a C++ compiler

CMakeLists.txt: Don't require a C++ compiler

By default, CMake assumes that the project is using both C and C++.  By
explicitly passing 'C' as argument of the project() macro, we tell CMake
that only C is used, which prevents CMake from erroring out if a C++
compiler doesn't exist.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
[Retrieved (and slightly updated by adding enable_language(CXX) if
WITH_ASTYLE is set) from:
https://git.buildroot.net/buildroot/tree/package/openjpeg/0003-CMakeLists.txt-Don-t-require-a-C-compiler.patch]
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>