util: Detect posix_fallocate() instead of assuming

Don't assume that `posix_fallocate()` is available on Linux and not
available on other operating systems. At least FreeBSD has it and we
are not using it.

Properly check whether `posix_fallocate()` is present and use it if it
is.

src/util/system.cpp

@@ -17,7 +17,7 @@
 #endif
 
 #ifndef WIN32
-// for posix_fallocate
+// for posix_fallocate, in configure.ac we check if it is present after this
 #ifdef __linux__
 
 #ifdef _POSIX_C_SOURCE
@@ -1012,7 +1012,7 @@ void AllocateFileRange(FILE *file, unsigned int offset, unsigned int length) {
     }
     ftruncate(fileno(file), static_cast<off_t>(offset) + length);
 #else
-    #if defined(__linux__)
+    #if defined(HAVE_POSIX_FALLOCATE)
     // Version using posix_fallocate
     off_t nEndPos = (off_t)offset + length;
     if (0 == posix_fallocate(fileno(file), 0, nEndPos)) return;