Fix various compiler warnings. (#1)

Those warnings arised depending on the PostgreSQL
version pg_checksum was built against, specifically
for the functions getControlFile() and syncDataDir(), which
is our own implementation for REL9_6_STABLE and REL_10_STABLE.

Also fix the compiler conditional for getControlFile(), the
version check for 9.6 has one digit too much.

pg_checksums.c

@@ -62,7 +62,7 @@ static void updateControlFile(char *DataDir, ControlFileData *ControlFile);
 #if PG_VERSION_NUM < 100000
 static void syncDataDir(char *DataDir, const char *argv0);
 #endif
-#if PG_VERSION_NUM < 906000
+#if PG_VERSION_NUM < 90600
 static ControlFileData *getControlFile(char *DataDir);
 #endif
 
@@ -268,6 +268,13 @@ scan_directory(char *basedir, char *subdir)
 	closedir(dir);
 }
 
+/*
+ * getControlFile() needs our own implementation in
+ * PostgreSQL versiones below 9.6. If built against newer version,
+ * use the get_controlfile() function provided there.
+ */
+#if PG_VERSION_NUM < 90600
+
 /*
  * Read in the control file.
  */
@@ -300,6 +307,8 @@ getControlFile(char *DataDir)
 	return ControlFile;
 }
 
+#endif
+
 /*
  * Update the control file.
  */
@@ -377,6 +386,11 @@ updateControlFile(char *DataDir, ControlFileData *ControlFile)
 	}
 }
 
+/*
+ * syncDataDir() is used on PostgreSQL releases <= 10, only
+ */
+#if PG_VERSION_NUM < 100000
+
 /*
  * Sync target data directory to ensure that modifications are safely on disk.
  *
@@ -431,6 +445,8 @@ syncDataDir(char *DataDir, const char *argv0)
 	}
 }
 
+#endif
+
 int
 main(int argc, char *argv[])
 {