support/cmake: impersonate a Linux system even more

Some packages test the system name to decide whether to enable/disable
features or link with specific libs.

So we forcefully set the system name form our custom system file, so
that packagses still believe they are running on Linux rather than
Buildroot.

Fixes:
    fastd      : http://autobuild.buildroot.net/results/f1d/f1dfe90068ad62e733f17a22202235415bda3974/
    paho-mqtt-c: http://autobuild.buildroot.net/results/457/457d76279e16247bf58c838a2c5dd0a4f3962c21/
    libiio     : http://autobuild.buildroot.net/results/281/2812b008a0ab6bab5fe4d45eb9ffe4e9496a8cb4/
    and so on...

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reported-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Ben Boeckel <mathstuf@gmail.com>
Cc: Jörg Krause <joerg.krause@embedded.rocks>
Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

support/misc/Buildroot.cmake

@@ -1,3 +1,7 @@
+# Impersonate a Linux system. Afterall, that's what we are...
+set(CMAKE_SYSTEM_NAME Linux)
 include(Platform/Linux)
+
+# Override problematic settings, to avoid RPATH against host lib directories.
 set_property(GLOBAL PROPERTY FIND_LIBRARY_USE_LIB32_PATHS FALSE)
 set_property(GLOBAL PROPERTY FIND_LIBRARY_USE_LIB64_PATHS FALSE)