cmake: check for non-empty env value, to not accept defined but empty ones

Author: vszakats

CMakeLists.txt

@@ -273,7 +273,7 @@ set(CURL_POST_INSTALL_COMMAND
 # Conditionally check to see if there's a source directory or not.
 # If there is, use it. Otherwise, download the latest version.
 #
-if(DEFINED ENV{CURL_SOURCE_DIR})
+if(NOT "$ENV{CURL_SOURCE_DIR}" STREQUAL "")
     message(STATUS "Building curl from source directory: $ENV{CURL_SOURCE_DIR}")
     ExternalProject_Add(
         curl_external
@@ -325,10 +325,10 @@ set(CURL_INCLUDE_DIRS
 set(CURL_LIB_DIR ${CURL_INSTALL_DIR}/lib)
 
 # Fuzzing engine
-if(DEFINED ENV{LIB_FUZZING_ENGINE})
+if(NOT "$ENV{LIB_FUZZING_ENGINE}" STREQUAL "")
     # Check to see if ENV{LIB_FUZZING_ENGINE} is a file. If so, use it directly.
     # Otherwise, assume it's a flag to the compiler.
-    if(EXISTS $ENV{LIB_FUZZING_ENGINE})
+    if(EXISTS "$ENV{LIB_FUZZING_ENGINE}")
         message(STATUS "Using LIB_FUZZING_ENGINE file: $ENV{LIB_FUZZING_ENGINE}")
         set(LIB_FUZZING_ENGINE $ENV{LIB_FUZZING_ENGINE})
         set(LIB_FUZZING_ENGINE_FLAG "")