How to check the OS with automake

I have a project that uses automake to create the configure and all related files (I’m using autoreconf command to make all this stuff). So, I’m trying to set some conditional files to compile when the project is compiling for macOS (OS X), Windows or Linux. But it fails with the following:

 $ autoreconf -i ..
src/Makefile.am:30: error: LINUX does not appear in AM_CONDITIONAL
autoreconf: automake failed with exit status: 1

And the part containing the error in that Makefile.am is the following:

if OSX
    butt_SOURCES += CurrentTrackOSX.h CurrentTrackOSX.m
endif
if LINUX
    butt_SOURCES += currentTrack.h currentTrackLinux.cpp
endif
if WINDOWS
    butt_SOURCES += currentTrack.h currentTrack.cpp
endif

My question is, how can I check if the OS is Linux? And if it’s possible, is there a better way to check the OS in automake?

Answer

You can detect it directly in the Makefile, or define the conditionals in the configure source file (probably configure.ac), since you are using autoreconf:

# AC_CANONICAL_HOST is needed to access the 'host_os' variable    
AC_CANONICAL_HOST

build_linux=no
build_windows=no
build_mac=no

# Detect the target system
case "${host_os}" in
    linux*)
        build_linux=yes
        ;;
    cygwin*|mingw*)
        build_windows=yes
        ;;
    darwin*)
        build_mac=yes
        ;;
    *)
        AC_MSG_ERROR(["OS $host_os is not supported"])
        ;;
esac

# Pass the conditionals to automake
AM_CONDITIONAL([LINUX], [test "$build_linux" = "yes"])
AM_CONDITIONAL([WINDOWS], [test "$build_windows" = "yes"])
AM_CONDITIONAL([OSX], [test "$build_mac" = "yes"])

Note: host_os refers to the target system, so if you are cross-compiling it sets the OS conditional of the system you are compiling to.