FLTK/CMakeLists.txt: fix CMAKE_MODULE_PATH, don't use 3rdparty code

Fix setting CMAKE_MODULE_PATH to respect the previous content of this
variable. Furthermore do not process subdirectories with 3rdparty code
even if no libjpeg, libpng or libz was found on the target system.

plugins/zynaddsubfx/fltk/CMakeLists.txt

@@ -44,7 +44,7 @@ set(LIBRARY_OUTPUT_PATH ${FLTK_BINARY_DIR}/lib)
 set(ARCHIVE_OUTPUT_PATH ${FLTK_BINARY_DIR}/lib)
 
 # Search for modules in the FLTK source dir first
-set(CMAKE_MODULE_PATH "${FLTK_SOURCE_DIR}/CMake")
+set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} ${FLTK_SOURCE_DIR}/CMake)
 
 include_directories(${FLTK_SOURCE_DIR} ${FLTK_BINARY_DIR})
 
@@ -469,7 +469,6 @@ if(OPTION_USE_SYSTEM_LIBJPEG AND JPEG_FOUND)
    include_directories(${JPEG_INCLUDE_DIR})
    set(FLTK_BUILTIN_JPEG_FOUND FALSE)
 else()
-   add_subdirectory(jpeg)
    set(FLTK_JPEG_LIBRARIES fltk_jpeg)
    include_directories(${FLTK_SOURCE_DIR}/jpeg)
    set(FLTK_BUILTIN_JPEG_FOUND TRUE)
@@ -493,7 +492,6 @@ if(OPTION_USE_SYSTEM_ZLIB AND ZLIB_FOUND)
    include_directories(${ZLIB_INCLUDE_DIRS})
    set(FLTK_BUILTIN_ZLIB_FOUND FALSE)
 else()
-   add_subdirectory(zlib)
    set(FLTK_ZLIB_LIBRARIES fltk_z)
    set(ZLIB_INCLUDE_DIR ${FLTK_SOURCE_DIR}/zlib)
    include_directories(${FLTK_SOURCE_DIR}/zlib)
@@ -519,7 +517,6 @@ if(OPTION_USE_SYSTEM_LIBPNG AND PNG_FOUND)
    add_definitions(${PNG_DEFINITIONS})
    set(FLTK_BUILTIN_PNG_FOUND FALSE)
 else()
-   add_subdirectory(png)
    set(FLTK_PNG_LIBRARIES fltk_png)
    set(HAVE_PNG_H 1)
    set(HAVE_PNG_GET_VALID 1)
@@ -590,7 +587,6 @@ if(CMAKE_CROSSCOMPILING)
       PROPERTIES IMPORTED_LOCATION ${FLUID_PATH}
       )
 else()
-#   add_subdirectory(fluid)
    set(FLTK_FLUID_EXECUTABLE fluid)
    set(FLTK_FLUID_PATH ${EXECUTABLE_OUTPUT_PATH}/${FLTK_FLUID_EXECUTABLE})
 endif(CMAKE_CROSSCOMPILING)