[build] add GNU system build matching

CMakeLists.txt

@@ -27,7 +27,8 @@ set_if(DARWIN      ${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
 set_if(LINUX       ${CMAKE_SYSTEM_NAME} MATCHES "Linux")
 set_if(BSD         ${SYSNAME_LC} MATCHES "bsd$")
 set_if(MICROSOFT   WIN32 AND (NOT MINGW AND NOT CYGWIN))
-set_if(SYMLINKABLE LINUX OR DARWIN OR BSD OR CYGWIN)
+set_if(GNU         ${CMAKE_SYSTEM_NAME} MATCHES "GNU")
+set_if(SYMLINKABLE LINUX OR DARWIN OR BSD OR CYGWIN OR GNU)
 
 # Not sure what to do in case of compiling by MSVC.
 # This will make installdir in C:\Program Files\SRT then
@@ -389,6 +390,9 @@ elseif(LINUX)
 elseif(CYGWIN)
 	add_definitions(-DCYGWIN=1)
 	message(STATUS "DETECTED SYSTEM: CYGWIN (posix mode); CYGWIN=1")
+elseif(GNU)
+	add_definitions(-DGNU=1)
+	message(STATUS "DETECTED SYSTEM: GNU;  GNU=1" )
 else()
 	message(FATAL_ERROR "Unsupported system: ${CMAKE_SYSTEM_NAME}")
 endif()