cmake: preserve CMAKE_REQUIRED_DEFINITIONS on BSD

The BSD block used set() to assign CMAKE_REQUIRED_DEFINITIONS,
which would silently discard any value already in that variable.
Switch to list(APPEND) so the BSD flag is accumulated rather than
replacing any pre-existing content.

Similarly, use list(APPEND) inside the check_symbol_exists block
when prepending the build flags, so the BSD definition set just
above is included in the probe compilations rather than overwritten.

Signed-off-by: Dan Nechita <dan.nechita@analog.com>

Author: dNechita

CMakeLists.txt

@@ -162,7 +162,7 @@ target_compile_definitions(iio PUBLIC
 
 if (${CMAKE_SYSTEM_NAME} MATCHES "FreeBSD|DragonFly|OpenBSD|NetBsd")
 	target_compile_definitions(iio PUBLIC __BSD_VISIBLE)
-	set(CMAKE_REQUIRED_DEFINITIONS -D__BSD_VISIBLE=1)
+	list(APPEND CMAKE_REQUIRED_DEFINITIONS -D__BSD_VISIBLE=1)
 endif()
 
 include(GNUInstallDirs)