CMakeLists.txt: Do not require C++ (#3956)

By default, the project() CMake command defaults to C and C++. [1]
Therefore, CMake might perform tests for both C and C++ compilers as
part of the configuration phase.

However, this has the consequence of the configuration phase to fail if
the system does not have a C++ toolchain installed, even if C++ is not
really used by the top-level project under the default settings.

Some configurations might still require a C++ toolchain, so
enable_language is selectively called under such circumstances.

[1]: https://cmake.org/cmake/help/latest/command/project.html

CMakeLists.txt

@@ -8,7 +8,7 @@ if(ESP_PLATFORM)
   return()
 endif()
 
-project (iwasm)
+project (iwasm LANGUAGES C)
 
 set(CMAKE_CXX_STANDARD 17)