OpenBSD: Fix system feature definitions

Since commit f034b0f663 (CMake compilation: do not use compiler
extensions, 2020-03-14, v3.18.0-rc1~494^2), some sources explicitly
enable needed system APIs on some platforms using definitions like
`_POSIX_C_SOURCE` and `_XOPEN_SOURCE`.  Drop the definitions for
OpenBSD, which provides the POSIX APIs by default.

Source/cmLoadCommandCommand.cxx

@@ -1,12 +1,12 @@
 /* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
    file Copyright.txt or https://cmake.org/licensing for details.  */
 
-#if !defined(_WIN32) && !defined(__sun)
+#if !defined(_WIN32) && !defined(__sun) && !defined(__OpenBSD__)
 // POSIX APIs are needed
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _POSIX_C_SOURCE 200809L
 #endif
-#if defined(__OpenBSD__) || defined(__FreeBSD__) || defined(__NetBSD__)
+#if defined(__FreeBSD__) || defined(__NetBSD__)
 // For isascii
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _XOPEN_SOURCE 700

Source/cmStandardLexer.h

@@ -7,7 +7,8 @@
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _XOPEN_SOURCE 600
 #endif
-#if !defined(_POSIX_C_SOURCE) && !defined(_WIN32) && !defined(__sun)
+#if !defined(_POSIX_C_SOURCE) && !defined(_WIN32) && !defined(__sun) &&       \
+  !defined(__OpenBSD__)
 /* POSIX APIs are needed */
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _POSIX_C_SOURCE 200809L
@@ -17,7 +18,7 @@
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _XOPEN_SOURCE 600
 #endif
-#if defined(__OpenBSD__) || defined(__FreeBSD__) || defined(__NetBSD__)
+#if defined(__FreeBSD__) || defined(__NetBSD__)
 /* For isascii */
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _XOPEN_SOURCE 700

Source/cmSystemTools.cxx

@@ -1,13 +1,12 @@
 /* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
    file Copyright.txt or https://cmake.org/licensing for details.  */
 
-#if !defined(_WIN32) && !defined(__sun)
+#if !defined(_WIN32) && !defined(__sun) && !defined(__OpenBSD__)
 // POSIX APIs are needed
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _POSIX_C_SOURCE 200809L
 #endif
-#if defined(__OpenBSD__) || defined(__FreeBSD__) || defined(__NetBSD__) ||    \
-  defined(__QNX__)
+#if defined(__FreeBSD__) || defined(__NetBSD__) || defined(__QNX__)
 // For isascii
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _XOPEN_SOURCE 700

Source/cmTimestamp.cxx

@@ -1,13 +1,12 @@
 /* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
    file Copyright.txt or https://cmake.org/licensing for details.  */
 
-#if !defined(_WIN32) && !defined(__sun)
+#if !defined(_WIN32) && !defined(__sun) && !defined(__OpenBSD__)
 // POSIX APIs are needed
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _POSIX_C_SOURCE 200809L
 #endif
-#if defined(__OpenBSD__) || defined(__FreeBSD__) || defined(__NetBSD__) ||    \
-  defined(__QNX__)
+#if defined(__FreeBSD__) || defined(__NetBSD__) || defined(__QNX__)
 // For isascii
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _XOPEN_SOURCE 700