Fix preprocessor logic that determines the availablity of strchrnul().

Apart from the error in the condition (&& should actually be ||), the
construct

    #if !defined(A) || !A

leads to a syntax error in the C preprocessor if A is indeed not defined.
Tested-by: NDavid Symonds <dsymonds@gmail.com>
Signed-off-by: NJohannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -183,7 +183,13 @@ void *gitmemmem(const void *haystack, size_t haystacklen,
                 const void *needle, size_t needlelen);
 #endif
 
-#if !defined(__GLIBC_PREREQ) && !__GLIBC_PREREQ(2, 1)
+#ifdef __GLIBC_PREREQ
+#if __GLIBC_PREREQ(2, 1)
+#define HAVE_STRCHRNUL
+#endif
+#endif
+
+#ifndef HAVE_STRCHRNUL
 #define strchrnul gitstrchrnul
 static inline char *gitstrchrnul(const char *s, int c)
 {