build: replace meson OS detection with variable

Some places were calling the meson function host_machine.system()
instead of the variables is_windows and is_linux defined
in config/meson.build.

At the same time, the missing "Linux restriction" reason is added to
pfe and octeontx2 crypto PMDs.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>

buildtools/pmdinfogen/meson.build

@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: BSD-3-Clause
 # Copyright(c) 2017 Intel Corporation
 
-if host_machine.system() == 'windows'
+if is_windows
 	subdir_done()
 endif
 

drivers/crypto/octeontx2/meson.build

@@ -1,8 +1,9 @@
 # SPDX-License-Identifier: BSD-3-Clause
 # Copyright (C) 2019 Marvell International Ltd.
 
-if host_machine.system() != 'linux'
-        build = false
+if not is_linux
+	build = false
+	reason = 'only supported on Linux'
 endif
 
 deps += ['bus_pci']

drivers/net/pfe/meson.build

@@ -1,8 +1,9 @@
 # SPDX-License-Identifier: BSD-3-Clause
 # Copyright 2019 NXP
 
-if host_machine.system() != 'linux'
+if not is_linux
 	build = false
+	reason = 'only supported on Linux'
 endif
 deps += ['common_dpaax']
 

examples/ntb/meson.build

@@ -7,7 +7,7 @@
 # DPDK instance, use 'make'
 
 allow_experimental_apis = true
-if host_machine.system() != 'linux'
+if not is_linux
 	build = false
 endif
 deps += 'rawdev'