CMake: allow to use build-in libraries on Linux

Requires updates of expat and tiff submodules.

Most libraries have their own CMakeLists, add a comment with a suggestion to
use this for implementing build-in support.

build/cmake/lib/expat.cmake

@@ -8,6 +8,8 @@
 #############################################################################
 
 if(wxUSE_EXPAT STREQUAL "builtin")
+    # TODO: implement building expat via its CMake file, using
+    # add_subdirectory or ExternalProject_Add
     wx_add_builtin_library(wxexpat
         src/expat/expat/lib/xmlparse.c
         src/expat/expat/lib/xmlrole.c

build/cmake/lib/png.cmake

@@ -8,6 +8,8 @@
 #############################################################################
 
 if(wxUSE_LIBPNG STREQUAL "builtin")
+    # TODO: implement building libpng via its CMake file, using
+    # add_subdirectory or ExternalProject_Add
     if(NOT MSVC)
         set(PNG_EXTRA_SOURCES
             src/png/mips/filter_msa_intrinsics.c

build/cmake/lib/regex.cmake

@@ -8,7 +8,6 @@
 #############################################################################
 
 if(wxUSE_REGEX)
-    # TODO: Forcing builtin until sys is implemented
     set(wxUSE_REGEX builtin)
     wx_add_builtin_library(wxregex
         src/regex/regcomp.c
@@ -20,5 +19,3 @@ if(wxUSE_REGEX)
     set(REGEX_LIBRARIES wxregex)
     set(REGEX_INCLUDE_DIRS ${wxSOURCE_DIR}/src/regex)
 endif()
-
-#TODO: find external lib and include dir

build/cmake/lib/tiff.cmake

@@ -8,13 +8,8 @@
 #############################################################################
 
 if(wxUSE_LIBTIFF STREQUAL "builtin")
-    # TODO: implement building libtiff via ExternalProject_Add()
-    if(UNIX AND NOT APPLE)
-        message(WARNING "Builtin libtiff on unix is currently not supported")
-        wx_option_force_value(wxUSE_LIBTIFF OFF)
-        return()
-    endif()
-
+    # TODO: implement building libtiff via its CMake file, using
+    # add_subdirectory or ExternalProject_Add
     if(WIN32)
         set(TIFF_PLATFORM_SRC src/tiff/libtiff/tif_win32.c)
     elseif(UNIX)

build/cmake/lib/zlib.cmake

@@ -8,6 +8,8 @@
 #############################################################################
 
 if(wxUSE_ZLIB STREQUAL "builtin")
+    # TODO: implement building zlib via its CMake file, using
+    # add_subdirectory or ExternalProject_Add
     wx_add_builtin_library(wxzlib
         src/zlib/adler32.c
         src/zlib/compress.c