Default compiler flags with Autotools

If the library you're checking for supplies a .pc file for use with pkg-config, then you're much better off using PKG_CHECK_MODULES to get the correct flags. libpng does:

(in configure.ac)

PKG_CHECK_MODULES([libpng], [libpng12])

This gives you access to the variables $(libpng_CFLAGS) and $(libpng_LIBS) which you will want to add to Makefile.am (probably in AM_CFLAGS/AM_CXXFLAGS and LDADD, or target-specific versions thereof).

It will also cause configure to fail with an error if libpng12.pc isn't found. If you want configure to continue, you'll need to supply the third and fourth arguments to PKG_CHECK_MODULES, which are ACTION-IF-FOUND and ACTION-IF-NOT-FOUND:

(in configure.ac)

PKG_CHECK_MODULES([libpng], [libpng12], [HAVE_LIBPNG=1], [HAVE_LIBPNG=0])

Now, if you need an automake conditional, you can do something like:

(in configure.ac)

AM_CONDITIONAL([USE_LIBPNG], [test "$HAVE_LIBPNG" -eq 1])

If you also need the preprocessor definition, you could use AC_DEFINE like so:

(in configure.ac)

AS_IF([test "$USE_LIBPNG" -eq 1], [AC_DEFINE([USE_LIBPNG], [1], [Define if using libpng.])])

Possibly nicer is to set the definition in Makefile.am:

(in Makefile.am)

AM_CPPFLAGS =
if USE_LIBPNG
AM_CPPFLAGS += -DUSE_LIBPNG
endif

This will clutter your command line, though, whereas AC_DEFINE can put the definition in a header if you use AC_CONFIG_HEADERS. I guess this doesn't really matter if you use AM_SILENT_RULES([yes]) or don't care about your command line being neat (and let's be honest, automake generates some pretty gnarly command lines anyway).

A note on good autoconf style

It is considered poor form to build optional support based on whether or not a check succeeded (see this gentoo doc for details). Here's how I'd code optional support for libpng:

(in configure.ac)

# This is because the first PKG_CHECK_MODULES call is inside a conditional.
PKG_PROG_PKG_CONFIG

AC_ARG_WITH([libpng],
  [AS_HELP_STRING([--with-libpng],
    [support handling png files @<:@default=check@:>@])],
  [],
  [with_libpng=check])
AS_CASE(["$with_libpng"],
  [yes], [PKG_CHECK_MODULES([libpng], [libpng12], [HAVE_LIBPNG=1])],
  [no], [],
  [PKG_CHECK_MODULES([libpng], [libpng12], [HAVE_LIBPNG=1], [HAVE_LIBPNG=0])])
AM_CONDITIONAL([USE_LIBPNG], [test "$with_libpng" != no -a "$HAVE_LIBPNG" -eq 1])

(in Makefile.am)

if USE_LIBPNG
AM_CPPFLAGS += -DUSE_LIBPNG
AM_CFLAGS += $(libpng_CFLAGS)
LDADD += $(libpng_LIBS)
libdev_la_SOURCES += png.c
endif

If your library doesn't have a .pc file

For completeness, here's how I'd check for a library that didn't have a .pc file. I'll skip over the details of following good autoconf style. AC_CHECK_LIB sets a cache variable, so you can test that instead of replacing the ACTION-IF-FOUND of AC_CHECK_LIB:

(in configure.ac)

AC_CHECK_LIB([png], [png_create_write_struct_2])

# Then test:
AS_IF([test "$ac_cv_lib_png_png_create_write_struct_2" = yes], [HAVE_LIBPNG=1], [HAVE_LIBPNG=0])

# Or set conditional:
AM_CONDITIONAL([USE_LIBPNG], [test "$ac_cv_lib_png_png_create_write_struct_2" = yes])

IMHO, you should only do it this way if you have no other option.