Check for missing inline on functions in public headers. (NVIDIA#2570)

* Check for missing `inline` on functions in public headers.

We can test for missing `inline` markup by linking the header objects into the same target twice.
By default CMake will eliminate duplicate link targets, but linking both the target and the $<TARGET_OBJECTS:target> bypasses the duplicate checks.

* Add -fPIC, formatting.

* Switch link_check to executable to avoid -fPIC issues

cmake/CCCLGenerateHeaderTests.cmake

@@ -108,4 +108,16 @@ function(cccl_generate_header_tests target_name project_include_path)
   add_library(${target_name} OBJECT ${header_srcs})
   cccl_configure_target(${target_name} ${cccl_configure_target_options})
 
+  # Check that all functions in headers are either template functions or inline:
+  set(link_target ${target_name}.link_check)
+  add_executable(${link_target} "${CCCL_SOURCE_DIR}/cmake/link_check_main.cpp")
+  cccl_configure_target(${link_target} ${cccl_configure_target_options})
+  # Linking both ${target_name} and $<TARGET_OBJECTS:${target_name}> forces CMake to
+  # link the same objects twice. The compiler will complain about duplicate symbols if
+  # any functions are missing inline markup.
+  target_link_libraries(${link_target} PRIVATE
+    ${target_name}
+    $<TARGET_OBJECTS:${target_name}>
+  )
+
 endfunction()

cmake/link_check_main.cpp

@@ -0,0 +1,4 @@
+int main()
+{
+  return 0;
+}