FindTerminfo.cmake: Don't require that C language support be enabled for CMake project.

[shaneharper]

Summary

Here's a simple CMakeLists.txt that demonstrates the problem:

project("My project does not use C" CXX)
set(LLVM_DIR  "${LLVM_obj-root}/lib/cmake/llvm")
find_package(LLVM REQUIRED CONFIG)

Test with:

cmake . -DLLVM_obj-root=`llvm-config --obj-root`

Without the patch to FindTerminfo.cmake CMake v3.16.3 will fail with:

Unknown extension ".c" for file
...
try_compile() works only for enabled languages.

[jackoalan]

Thank you for the patch! It makes sense that the check functions depend on the corresponding language to be enabled.

The only issue I see is the converse is also potentially true (where the importer only has C enabled to link with the C API).

It should be written to check C if it is in the ENABLED_LANGUAGES global property, and fall back to C++ otherwise.

If you could also make the same modification to FindFFI.cmake, this will be a very useful change for importers with C++-only projects.

llvm/cmake/modules/FindTerminfo.cmake

 
 if(Terminfo_LIBRARIES)
   include(CMakePushCheckState)
-  include(CheckCSourceCompiles)
+  include(CheckCXXSourceCompiles)
   cmake_push_check_state()
   list(APPEND CMAKE_REQUIRED_LIBRARIES ${Terminfo_LIBRARIES})
-  check_c_source_compiles("
-    int setupterm(char *term, int filedes, int *errret);
+  check_cxx_source_compiles("
+    extern \"C\" { int setupterm(char *term, int filedes, int *errret); }
     int main() { return setupterm(0, 0, 0); }"
     Terminfo_LINKABLE)
   cmake_pop_check_state()