add func_attr.h to be able to use func attribs

Currently enables these for every gnu-like compilers, i.e.: gcc, clang and
icc.

src/func_attr.h

@@ -0,0 +1,58 @@
+#ifndef NEOVIM_FUNC_ATTR_H
+#define NEOVIM_FUNC_ATTR_H
+
+// gcc and clang expose their version as follows:
+//
+// gcc 4.7.2:
+//   __GNUC__          = 4
+//   __GNUC_MINOR__    = 7
+//   __GNUC_PATCHLEVEL = 2
+//
+// clang 3.4 (claims compat with gcc 4.2.1):
+//   __GNUC__          = 4
+//   __GNUC_MINOR__    = 2
+//   __GNUC_PATCHLEVEL = 1
+//   __clang__         = 1
+//   __clang_major__   = 3
+//   __clang_minor__   = 4
+//
+// To view the default defines of these compilers, you can perform:
+//
+// $ gcc -E -dM - </dev/null
+// $ echo | clang -dM -E -
+
+#ifdef __GNUC__
+  // place defines for all gnulikes here, for now that's gcc, clang and
+  // intel.
+
+  // place these after the argument list of the function declaration
+  // (not definition), like so:
+  // void myfunc(void) FUNC_ATTR_ALWAYS_INLINE;
+  #define FUNC_ATTR_MALLOC __attribute__((malloc))
+  #define FUNC_ATTR_ALLOC_SIZE(x) __attribute__((alloc_size(x)))
+  #define FUNC_ATTR_ALLOC_SIZE_PROD(x,y) __attribute__((alloc_size(x,y)))
+  #define FUNC_ATTR_ALLOC_ALIGN(x) __attribute__((alloc_align(x)))
+  #define FUNC_ATTR_PURE __attribute__ ((pure))
+  #define FUNC_ATTR_CONST __attribute__((const))
+  #define FUNC_ATTR_WARN_UNUSED_RESULT __attribute__((warn_unused_result))
+  #define FUNC_ATTR_ALWAYS_INLINE __attribute__((always_inline))
+
+  #ifdef __clang__
+    // clang only
+  #elif defined(__INTEL_COMPILER)
+    // intel only
+  #else
+    // gcc only
+  #endif
+#else
+  #define FUNC_ATTR_MALLOC
+  #define FUNC_ATTR_ALLOC_SIZE(x)
+  #define FUNC_ATTR_ALLOC_SIZE_PROD(x,y)
+  #define FUNC_ATTR_ALLOC_ALIGN(x)
+  #define FUNC_ATTR_PURE
+  #define FUNC_ATTR_CONST
+  #define FUNC_ATTR_WARN_UNUSED_RESULT
+  #define FUNC_ATTR_ALWAYS_INLINE
+#endif
+
+#endif // NEOVIM_FUNC_ATTR_H