[CMake] config.h, part 2 - full compatibility with autoconf.

This commit makes the file config.h generated by CMake 100% compatible
with the one generated by autoconf/configure.

Fixes in this commit:
 - Set FLTK_DATADIR and FLTK_DOCDIR with the same default values.
   Note: needs some cleanup, option values are not clear, '/fltk'
   is currently appended in export.cmake (needs improvement).
 - Simulation of autoconf macro AC_HEADER_DIRENT for correct
   definition of only one 'dirent' header file.
 - Fix more function checks:
    - dlsym
    - png_get_valid
    - png_set_tRNS_to_alpha
 - Improve "Big Endian" check for __APPLE__ (mac_endianness.h).


git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@10985 ea41ed52-d2ee-0310-a9c1-e6b18d33e121

configh.cmake.in

@@ -1,8 +1,8 @@
+/* @CONFIG_H@.  Generated from @CONFIG_H_IN@ by CMake.  */
 /*
  * "$Id$"
  *
  * Configuration file for the Fast Light Tool Kit (FLTK).
- * @configure_input@
  *
  * Copyright 1998-2015 by Bill Spitzak and others.
  *
@@ -21,8 +21,8 @@
  * Where to find files...
  */
 
-#define FLTK_DATADIR	"@PREFIX_DATA@"
-#define FLTK_DOCDIR	"@PREFIX_DOC@"
+#define FLTK_DATADIR "@PREFIX_DATA@"
+#define FLTK_DOCDIR "@PREFIX_DOC@"
 
 /*
  * BORDER_WIDTH:
@@ -147,7 +147,7 @@
  * no meaning on operating systems other than Mac OS X.
  */
 
-#cmakedefine __APPLE_QUARTZ__ @__APPLE_QUARTZ__@
+#cmakedefine __APPLE_QUARTZ__ 1
 
 
 /*
@@ -185,7 +185,11 @@
  * Byte order of your machine: 1 = big-endian, 0 = little-endian.
  */
 
-#define WORDS_BIGENDIAN @WORDS_BIGENDIAN@
+#ifdef __APPLE__
+#include <mac_endianness.h>
+#else
+#cmakedefine01 WORDS_BIGENDIAN
+#endif
 
 /*
  * U16, U32, U64: