treewide: avoid use of `inline` attribute

ISO C90 does not specify the `inline` attribute, and as such we cannot
use it in our code. While we already use `__inline` when building in
Microsoft Visual Studio, we should also be using the `__inline__`
attribute from GCC/Clang. Otherwise, if we're using neither MSVC nor
GCC/Clang, we should simply avoid using `inline` at all and just define
functions as static.

This commit adjusts our own `GIT_INLINE` macro as well as the inline
macros specified by khash and xdiff. This allows us to enable strict C90
mode in a later commit.

src/common.h

@@ -17,8 +17,10 @@
 /** Declare a function as always inlined. */
 #if defined(_MSC_VER)
 # define GIT_INLINE(type) static __inline type
+#elif defined(__GNUC__)
+# define GIT_INLINE(type) static __inline__ type
 #else
-# define GIT_INLINE(type) static inline type
+# define GIT_INLINE(type) static type
 #endif
 
 /** Support for gcc/clang __has_builtin intrinsic */

src/khash.h

@@ -146,8 +146,10 @@ typedef unsigned long long khint64_t;
 #ifndef kh_inline
 #ifdef _MSC_VER
 #define kh_inline __inline
+#elif defined(__GNUC__)
+#define kh_inline __inline__
 #else
-#define kh_inline inline
+#define kh_inline
 #endif
 #endif /* kh_inline */
 

src/xdiff/xdiffi.c

@@ -33,8 +33,10 @@
 /** Declare a function as always inlined. */
 #if defined(_MSC_VER)
 # define XDL_INLINE(type) static __inline type
+#elif defined(__GNUC__)
+# define XDL_INLINE(type) static __inline__ type
 #else
-# define XDL_INLINE(type) static inline type
+#define XDG_INLINE(type) static type
 #endif
 
 typedef struct s_xdpsplit {