allow inclusion in C programs (#4608)

* allow inclusion in C programs

* adding documentation to macro

* Support for ANSI C, _Thread_local where available.

* fix macro for docs

include/LightGBM/c_api.h

@@ -15,9 +15,16 @@
 
 #include <LightGBM/export.h>
 
+#ifdef __cplusplus
 #include <cstdint>
 #include <cstdio>
 #include <cstring>
+#else
+#include <stdint.h>
+#include <stdio.h>
+#include <string.h>
+#include <stdbool.h>
+#endif
 
 
 typedef void* DatasetHandle;  /*!< \brief Handle of dataset. */
@@ -79,7 +86,7 @@ LIGHTGBM_C_EXPORT int LGBM_SampleIndices(int32_t num_total_row,
                                          void* out,
                                          int32_t* out_len);
 
-// --- start Dataset interface
+/* --- start Dataset interface */
 
 /*!
  * \brief Load dataset from file (like LightGBM CLI version does).
@@ -424,7 +431,7 @@ LIGHTGBM_C_EXPORT int LGBM_DatasetGetNumFeature(DatasetHandle handle,
 LIGHTGBM_C_EXPORT int LGBM_DatasetAddFeaturesFrom(DatasetHandle target,
                                                   DatasetHandle source);
 
-// --- start Booster interfaces
+/* --- start Booster interfaces */
 
 /*!
 * \brief Get boolean representing whether booster is fitting linear trees.
@@ -1321,7 +1328,17 @@ LIGHTGBM_C_EXPORT int LGBM_NetworkInitWithFunctions(int num_machines,
                                                     void* reduce_scatter_ext_fun,
                                                     void* allgather_ext_fun);
 
-#if defined(_MSC_VER)
+#if !defined(__cplusplus) && (!defined(__STDC__) || (__STDC_VERSION__ < 199901L))
+#define INLINE_FUNCTION /*!< \brief inline specifier no-op in C using standards before C99. */
+#else
+#define INLINE_FUNCTION inline /*!< \brief Inline specifier. */
+#endif
+
+#if !defined(__cplusplus) && (!defined(__STDC__) || (__STDC_VERSION__ < 201112L))
+#define THREAD_LOCAL  /*!< \brief Thread local specifier no-op in C using standards before C11. */
+#elif !defined(__cplusplus)
+#define THREAD_LOCAL _Thread_local  /*!< \brief Thread local specifier. */
+#elif defined(_MSC_VER)
 #define THREAD_LOCAL __declspec(thread)  /*!< \brief Thread local specifier. */
 #else
 #define THREAD_LOCAL thread_local  /*!< \brief Thread local specifier. */
@@ -1340,9 +1357,9 @@ static char* LastErrorMsg() { static THREAD_LOCAL char err_msg[512] = "Everythin
  * \brief Set string message of the last error.
  * \param msg Error message
  */
-inline void LGBM_SetLastError(const char* msg) {
+INLINE_FUNCTION void LGBM_SetLastError(const char* msg) {
   const int err_buf_len = 512;
   snprintf(LastErrorMsg(), err_buf_len, "%s", msg);
 }
 
-#endif  // LIGHTGBM_C_API_H_
+#endif  /* LIGHTGBM_C_API_H_ */