fix gmp setup

third_party/CMakeLists.txt

@@ -16,6 +16,27 @@
 
 include(ExternalProject)
 
+# GMP
+# declaring it first to be used by intx, libff
+if(DEFINED GMP_LIBRARY)
+  add_library(gmplib STATIC IMPORTED)
+  set_target_properties(gmplib PROPERTIES IMPORTED_LOCATION "${GMP_LIBRARY}")
+  if(DEFINED GMP_INCLUDE_DIR)
+    set_target_properties(gmplib PROPERTIES INTERFACE_INCLUDE_DIRECTORIES "${GMP_INCLUDE_DIR}")
+  else()
+    message(FATAL_ERROR "GMP_INCLUDE_DIR is required if a custom GMP_LIBRARY is provided")
+  endif()
+else()
+  find_package(gmp REQUIRED)
+  add_custom_target(gmplib DEPENDS gmp::GMP)
+
+  # derive the path variables from the package for intx, libff
+  get_target_property(GMP_INCLUDE_DIR gmp::GMP INTERFACE_INCLUDE_DIRECTORIES)
+  set(GMP_INCLUDE_DIR "${GMP_INCLUDE_DIR}" CACHE PATH "")
+  get_target_property(GMP_LIBRARY_DIR gmp::GMP INTERFACE_LINK_DIRECTORIES)
+  find_library(GMP_LIBRARY gmp "${GMP_LIBRARY_DIR}")
+endif()
+
 # evmone with dependencies
 add_subdirectory(intx)
 
@@ -69,18 +90,6 @@ else()
   target_compile_options(evmone PRIVATE -fno-exceptions)
 endif()
 
-# GMP
-if(DEFINED GMP_LIBRARY)
-  add_library(gmplib STATIC IMPORTED)
-  set_target_properties(gmplib PROPERTIES IMPORTED_LOCATION "${GMP_LIBRARY}")
-  if(DEFINED GMP_INCLUDE_DIR)
-    set_target_properties(gmplib PROPERTIES INTERFACE_INCLUDE_DIRECTORIES "${GMP_INCLUDE_DIR}")
-  endif()
-else()
-  find_package(gmp REQUIRED)
-  add_custom_target(gmplib DEPENDS gmp::gmp)
-endif()
-
 # secp256k1
 add_library(secp256k1 secp256k1/src/secp256k1.c)
 if(MSVC)