CMake: only do jpeg-turbo tests if a libjpeg was found

CMakeLists.txt

@@ -58,39 +58,41 @@ endif(WITH_ZLIB)
 
 if(WITH_JPEG)
   find_package(JPEG)
-  # Check whether the version of libjpeg we found was libjpeg-turbo and print a
-  # warning if not.
-  set(CMAKE_REQUIRED_LIBRARIES ${JPEG_LIBRARIES})
-  set(CMAKE_REQUIRED_FLAGS -I${JPEG_INCLUDE_DIR})
-
-  set(JPEG_TEST_SOURCE "\n
-  #include <stdio.h>\n
-  #include <jpeglib.h>\n
-  int main(void) {\n
-    struct jpeg_compress_struct cinfo;\n
-    struct jpeg_error_mgr jerr;\n
-    cinfo.err=jpeg_std_error(&jerr);\n
-    jpeg_create_compress(&cinfo);\n
-    cinfo.input_components = 3;\n
-    jpeg_set_defaults(&cinfo);\n
-    cinfo.in_color_space = JCS_EXT_RGB;\n
-    jpeg_default_colorspace(&cinfo);\n
-    return 0;\n
-  }")
-
-  if(CMAKE_CROSSCOMPILING)
-    check_c_source_compiles("${JPEG_TEST_SOURCE}" FOUND_LIBJPEG_TURBO)
-  else()
-    check_c_source_runs("${JPEG_TEST_SOURCE}" FOUND_LIBJPEG_TURBO)
-  endif()
-
-  set(CMAKE_REQUIRED_LIBRARIES)
-  set(CMAKE_REQUIRED_FLAGS)
-  set(CMAKE_REQUIRED_DEFINITIONS)
-
-  if(NOT FOUND_LIBJPEG_TURBO)
-    message(WARNING "*** The libjpeg library you are building against is not libjpeg-turbo.  Performance will be reduced.  You can obtain libjpeg-turbo from:  https://sourceforge.net/projects/libjpeg-turbo/files/ ***")
-  endif()
+  if(JPEG_FOUND)
+    # Check whether the version of libjpeg we found was libjpeg-turbo and print a
+    # warning if not.
+    set(CMAKE_REQUIRED_LIBRARIES ${JPEG_LIBRARIES})
+    set(CMAKE_REQUIRED_FLAGS -I${JPEG_INCLUDE_DIR})
+
+    set(JPEG_TEST_SOURCE "\n
+    #include <stdio.h>\n
+    #include <jpeglib.h>\n
+    int main(void) {\n
+      struct jpeg_compress_struct cinfo;\n
+      struct jpeg_error_mgr jerr;\n
+      cinfo.err=jpeg_std_error(&jerr);\n
+      jpeg_create_compress(&cinfo);\n
+      cinfo.input_components = 3;\n
+      jpeg_set_defaults(&cinfo);\n
+      cinfo.in_color_space = JCS_EXT_RGB;\n
+      jpeg_default_colorspace(&cinfo);\n
+      return 0;\n
+    }")
+
+    if(CMAKE_CROSSCOMPILING)
+      check_c_source_compiles("${JPEG_TEST_SOURCE}" FOUND_LIBJPEG_TURBO)
+    else()
+      check_c_source_runs("${JPEG_TEST_SOURCE}" FOUND_LIBJPEG_TURBO)
+    endif()
+
+    set(CMAKE_REQUIRED_LIBRARIES)
+    set(CMAKE_REQUIRED_FLAGS)
+    set(CMAKE_REQUIRED_DEFINITIONS)
+
+    if(NOT FOUND_LIBJPEG_TURBO)
+      message(WARNING "*** The libjpeg library you are building against is not libjpeg-turbo.  Performance will be reduced.  You can obtain libjpeg-turbo from:  https://sourceforge.net/projects/libjpeg-turbo/files/ ***")
+    endif()
+  endif(JPEG_FOUND)
 endif(WITH_JPEG)