Use defined(__GNUC__) before __GNUC__ in preprocessor lines.

This should silence the equivalent of -Wundef in compilers that
don't define __GNUC__.

src/common/sysdefs.h

@@ -189,7 +189,8 @@
 #	define ARRAY_SIZE(array) (sizeof(array) / sizeof((array)[0]))
 #endif
 
-#if (__GNUC__ == 4 && __GNUC_MINOR__ >= 3) || __GNUC__ > 4
+#if defined(__GNUC__) \
+		&& ((__GNUC__ == 4 && __GNUC_MINOR__ >= 3) || __GNUC__ > 4)
 #	define lzma_attr_alloc_size(x) __attribute__((__alloc_size__(x)))
 #else
 #	define lzma_attr_alloc_size(x)

src/liblzma/api/lzma.h

@@ -224,7 +224,8 @@
 #		else
 #			define lzma_nothrow throw()
 #		endif
-#	elif __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 3)
+#	elif defined(__GNUC__) && (__GNUC__ > 3 \
+			|| (__GNUC__ == 3 && __GNUC_MINOR__ >= 3))
 #		define lzma_nothrow __attribute__((__nothrow__))
 #	else
 #		define lzma_nothrow
@@ -241,7 +242,7 @@
  * break anything if these are sometimes enabled and sometimes not, only
  * affects warnings and optimizations.
  */
-#if __GNUC__ >= 3
+#if defined(__GNUC__) && __GNUC__ >= 3
 #	ifndef lzma_attribute
 #		define lzma_attribute(attr) __attribute__(attr)
 #	endif