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automake-1.16: Program Variables

 
 8.7 Variables used when building a program
 ==========================================
 
 Occasionally it is useful to know which ‘Makefile’ variables Automake
 uses for compilations, and in which order (⇒Flag Variables
 Ordering); for instance, you might need to do your own compilation in
 some special cases.
 
    Some variables are inherited from Autoconf; these are ‘CC’, ‘CFLAGS’,
 ‘CPPFLAGS’, ‘DEFS’, ‘LDFLAGS’, and ‘LIBS’.
 
    There are some additional variables that Automake defines on its own:
 
 ‘AM_CPPFLAGS’
      The contents of this variable are passed to every compilation that
      invokes the C preprocessor; it is a list of arguments to the
      preprocessor.  For instance, ‘-I’ and ‘-D’ options should be listed
      here.
 
      Automake already provides some ‘-I’ options automatically, in a
      separate variable that is also passed to every compilation that
      invokes the C preprocessor.  In particular it generates ‘-I.’,
      ‘-I$(srcdir)’, and a ‘-I’ pointing to the directory holding
      ‘config.h’ (if you’ve used ‘AC_CONFIG_HEADERS’).  You can disable
      the default ‘-I’ options using the ‘nostdinc’ option.
 
      When a file to be included is generated during the build and not
      part of a distribution tarball, its location is under
      ‘$(builddir)’, not under ‘$(srcdir)’.  This matters especially for
      packages that use header files placed in sub-directories and want
      to allow builds outside the source tree (⇒VPATH Builds).  In
      that case we recommend using a pair of ‘-I’ options, such as, e.g.,
      ‘-Isome/subdir -I$(srcdir)/some/subdir’ or
      ‘-I$(top_builddir)/some/subdir -I$(top_srcdir)/some/subdir’.  Note
      that the reference to the build tree should come before the
      reference to the source tree, so that accidentally leftover
      generated files in the source directory are ignored.
 
      ‘AM_CPPFLAGS’ is ignored in preference to a per-executable (or
      per-library) ‘_CPPFLAGS’ variable if it is defined.
 
 ‘INCLUDES’
      This does the same job as ‘AM_CPPFLAGS’ (or any per-target
      ‘_CPPFLAGS’ variable if it is used).  It is an older name for the
      same functionality.  This variable is deprecated; we suggest using
      ‘AM_CPPFLAGS’ and per-target ‘_CPPFLAGS’ instead.
 
 ‘AM_CFLAGS’
      This is the variable the ‘Makefile.am’ author can use to pass in
      additional C compiler flags.  In some situations, this is not used,
      in preference to the per-executable (or per-library) ‘_CFLAGS’.
 
 ‘COMPILE’
      This is the command used to compile a C source file.  The file name
      is appended to form the complete command line.
 
 ‘AM_LDFLAGS’
      This is the variable the ‘Makefile.am’ author can use to pass in
      additional linker flags.  In some situations, this is not used, in
      preference to the per-executable (or per-library) ‘_LDFLAGS’.
 
 ‘LINK’
      This is the command used to link a C program.  It already includes
      ‘-o $@’ and the usual variable references (for instance, ‘CFLAGS’);
      it takes as “arguments” the names of the object files and libraries
      to link in.  This variable is not used when the linker is
      overridden with a per-target ‘_LINK’ variable or per-target flags
      cause Automake to define such a ‘_LINK’ variable.
 

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