diff --git a/FindPreproc.cmake b/FindPreproc.cmake
index cffb441..67b159d 100644
--- a/FindPreproc.cmake
+++ b/FindPreproc.cmake
@@ -1,5 +1,5 @@
-if(NOT LIBPREPROC_PREFIX STREQUAL CMAKE_BINARY_DIR)
-  set(LIBPREPROC_PREFIX "" CACHE PATH "The path to the previx of an libpreproc installation")
+if(NOT LIBPREPROC_IS_EXTERNAL_PROJECT)
+  set(LIBPREPROC_PREFIX "" CACHE PATH "The path to the prefix of an libpreproc installation")
 
   find_path(
     LIBPREPROC_INCLUDE_DIRS
diff --git a/FindPreprocmath.cmake b/FindPreprocmath.cmake
index 3150c96..70d46f0 100644
--- a/FindPreprocmath.cmake
+++ b/FindPreprocmath.cmake
@@ -1,5 +1,5 @@
-if(NOT LIBPREPROCMATH_PREFIX STREQUAL CMAKE_BINARY_DIR)
-  set(LIBPREPROCMATH_PREFIX "" CACHE PATH "The path to the previx of an libpreprocmath installation")
+if(NOT LIBPREPROCMATH_IS_EXTERNAL_PROJECT)
+  set(LIBPREPROCMATH_PREFIX "" CACHE PATH "The path to the prefix of an libpreprocmath installation")
 
   find_path(
     LIBPREPROCMATH_INCLUDE_DIRS