revise iconv detection:

- check libiconv with a linkage test with iconv.h included
- check libc iconv with a linkage test with iconv.h included
  and LIBICONV_PLUG defined (in case libiconv header is in
  include path)
- add new configuration option to prefer iconv from libiconv,
  if available, over the libc version: SDL_LIBICONV, defaults
  to disabled.
- remove FindIconv + pkg_check_modules for iconv, and use our
  manual iconv finding only
- change FreeBSD specific LIBICONV_PLUG define in SDL_iconv.c
  to configuration result.

include/build_config/SDL_build_config.h.cmake

@@ -196,6 +196,7 @@
 #cmakedefine HAVE_CLOCK_GETTIME 1
 #cmakedefine HAVE_GETPAGESIZE 1
 #cmakedefine HAVE_ICONV 1
+#cmakedefine SDL_USE_LIBICONV 1
 #cmakedefine HAVE_PTHREAD_SETNAME_NP 1
 #cmakedefine HAVE_PTHREAD_SET_NAME_NP 1
 #cmakedefine HAVE_SEM_TIMEDWAIT 1
@@ -223,7 +224,7 @@
 
 #cmakedefine HAVE_LINUX_INPUT_H 1
 #cmakedefine HAVE_LIBUDEV_H 1
-#cmakedefine HAVE_LIBDECOR_H  1
+#cmakedefine HAVE_LIBDECOR_H 1
 
 #cmakedefine HAVE_D3D_H @HAVE_D3D_H@
 #cmakedefine HAVE_D3D11_H @HAVE_D3D11_H@
@@ -487,7 +488,7 @@
 #cmakedefine SDL_ARM_NEON_BLITTERS @SDL_ARM_NEON_BLITTERS@
 
 /* Whether SDL_DYNAMIC_API needs dlopen */
-#cmakedefine DYNAPI_NEEDS_DLOPEN  @DYNAPI_NEEDS_DLOPEN@
+#cmakedefine DYNAPI_NEEDS_DLOPEN @DYNAPI_NEEDS_DLOPEN@
 
 /* Enable ime support */
 #cmakedefine SDL_USE_IME @SDL_USE_IME@