Avoid inline Keyword in C90

Previously we would use it for all gcc-like compilations, even when a
restrictive mode that disallowed it had been selected.
diff --git a/lib/common/compiler.h b/lib/common/compiler.h
index 2f18efb..95e9483 100644
--- a/lib/common/compiler.h
+++ b/lib/common/compiler.h
@@ -17,7 +17,7 @@
 /* force inlining */
 
 #if !defined(ZSTD_NO_INLINE)
-#if defined (__GNUC__) || defined(__cplusplus) || defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L   /* C99 */
+#if (defined(__GNUC__) && !defined(__STRICT_ANSI__)) || defined(__cplusplus) || defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L   /* C99 */
 #  define INLINE_KEYWORD inline
 #else
 #  define INLINE_KEYWORD