(autoconf.info)System Services

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4.9 System Services
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The following macros check for operating system services or
capabilities.

 -- Macro: AC_CYGWIN
     Checks for the Cygwin environment.  If present, sets shell variable
     'CYGWIN' to 'yes'.  If not present, sets 'CYGWIN' to the empty
     string.

 -- Macro: AC_EXEEXT
     Defines substitute variable 'EXEEXT' based on the output of the
     compiler, after .c, .o, and .obj files have been excluded.
     Typically set to empty string if Unix, '.exe' or '.EXE' if Win32.

 -- Macro: AC_OBJEXT
     Defines substitute variable 'OBJEXT' based on the output of the
     compiler, after .c files have been excluded.  Typically set to '.o'
     if Unix, '.obj' if Win32.

 -- Macro: AC_MINGW32
     Checks for the MingW32 compiler environment.  If present, sets
     shell variable 'MINGW32' to 'yes'.  If not present, sets 'MINGW32'
     to the empty string.

 -- Macro: AC_PATH_X
     Try to locate the X Window System include files and libraries.  If
     the user gave the command line options '--x-includes=DIR' and
     '--x-libraries=DIR', use those directories.  If either or both were
     not given, get the missing values by running 'xmkmf' on a trivial
     'Imakefile' and examining the 'Makefile' that it produces.  If that
     fails (such as if 'xmkmf' is not present), look for them in several
     directories where they often reside.  If either method is
     successful, set the shell variables 'x_includes' and 'x_libraries'
     to their locations, unless they are in directories the compiler
     searches by default.

     If both methods fail, or the user gave the command line option
     '--without-x', set the shell variable 'no_x' to 'yes'; otherwise
     set it to the empty string.

 -- Macro: AC_PATH_XTRA
     An enhanced version of 'AC_PATH_X'.  It adds the C compiler flags
     that X needs to output variable 'X_CFLAGS', and the X linker flags
     to 'X_LIBS'.  If X is not available, adds '-DX_DISPLAY_MISSING' to
     'X_CFLAGS'.

     This macro also checks for special libraries that some systems need
     in order to compile X programs.  It adds any that the system needs
     to output variable 'X_EXTRA_LIBS'.  And it checks for special X11R6
     libraries that need to be linked with before '-lX11', and adds any
     found to the output variable 'X_PRE_LIBS'.

 -- Macro: AC_SYS_INTERPRETER
     Check whether the system supports starting scripts with a line of
     the form '#! /bin/csh' to select the interpreter to use for the
     script.  After running this macro, shell code in 'configure.in' can
     check the shell variable 'interpval'; it will be set to 'yes' if
     the system supports '#!', 'no' if not.

 -- Macro: AC_SYS_LONG_FILE_NAMES
     If the system supports file names longer than 14 characters, define
     'HAVE_LONG_FILE_NAMES'.

 -- Macro: AC_SYS_RESTARTABLE_SYSCALLS
     If the system automatically restarts a system call that is
     interrupted by a signal, define 'HAVE_RESTARTABLE_SYSCALLS'.