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Conditional Compiler Flag Setting

Sometimes we want to only set a particular set of compiler flags if a particular header file or library is present. 

AM_CONDITIONAL(HAVE_CDDA, false)
AC_CHECK_HEADERS(cdda_interface.h cdda_paranoia.h, [
        CDDA_LIBS="-lcdda_paranoia -lcdda_interface"
        AM_CONDITIONAL(HAVE_CDDA, true)
        ])

Another, more complicated example. 

AC_CHECK_LIB(jpeg, jpeg_start_decompress,
  [AC_CHECK_HEADER(jpeglib.h,
    jpeg_ok=yes,
    jpeg_ok=no)],
  AC_MSG_WARN(*** (jpeg library not found) ***), -lm)

if test "$jpeg_ok" = yes; then
  JPEG='jpeg'; LIBJPEG='-ljpeg'
  AC_DEFINE(HAVE_LIBJPEG)
else
 AC_MSG_WARN(*** JPEG loader will not be built ***)
fi
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