Propagate CMake flags to try_compile (fixes #1900) (#1905)

* Forward CMake environment flags to libjpeg tests

* Forward CMake environment flags to libpng tests

dlib/cmake_utils/find_libjpeg.cmake

@@ -16,11 +16,17 @@ endif()
 
 find_package(JPEG QUIET)
 
-if(JPEG_FOUND) 
+if(JPEG_FOUND)
+   set(JPEG_TEST_CMAKE_FLAGS 
+      "-DCMAKE_PREFIX_PATH=${CMAKE_PREFIX_PATH}"
+      "-DCMAKE_INCLUDE_PATH=${CMAKE_INCLUDE_PATH}"
+      "-DCMAKE_LIBRARY_PATH=${CMAKE_LIBRARY_PATH}")
+
    try_compile(test_for_libjpeg_worked 
       ${PROJECT_BINARY_DIR}/test_for_libjpeg_build  
       ${CMAKE_CURRENT_LIST_DIR}/test_for_libjpeg
-      test_if_libjpeg_is_broken)
+      test_if_libjpeg_is_broken
+      CMAKE_FLAGS "${JPEG_TEST_CMAKE_FLAGS}")
 
    message (STATUS "Found system copy of libjpeg: ${JPEG_LIBRARY}")
    if(NOT test_for_libjpeg_worked)

dlib/cmake_utils/find_libpng.cmake

@@ -16,11 +16,17 @@ endif()
 
 find_package(PNG QUIET)
 
-if(PNG_FOUND) 
+if(PNG_FOUND)
+   set(PNG_TEST_CMAKE_FLAGS 
+      "-DCMAKE_PREFIX_PATH=${CMAKE_PREFIX_PATH}"
+      "-DCMAKE_INCLUDE_PATH=${CMAKE_INCLUDE_PATH}"
+      "-DCMAKE_LIBRARY_PATH=${CMAKE_LIBRARY_PATH}")
+
    try_compile(test_for_libpng_worked 
       ${PROJECT_BINARY_DIR}/test_for_libpng_build  
       ${CMAKE_CURRENT_LIST_DIR}/test_for_libpng
-      test_if_libpng_is_broken)
+      test_if_libpng_is_broken
+      CMAKE_FLAGS "${PNG_TEST_CMAKE_FLAGS}")
 
    message (STATUS "Found system copy of libpng: ${PNG_LIBRARIES}")
    if(NOT test_for_libpng_worked)