test: fix search of optional dependencies

unittest/CMakeLists.txt

@@ -136,17 +136,15 @@ IF(BUILD_ADVANCED_TESTING)
   ADD_PINOCCHIO_UNIT_TEST(multiprecision)
   SET_PROPERTY(TARGET test-cpp-multiprecision PROPERTY CXX_STANDARD 11)
 
-  find_package(PkgConfig QUIET)
-  IF(PkgConfig_FOUND)
-    pkg_check_modules(MPFR QUIET mpfr)
-    IF(MPFR_FOUND)
-      ADD_PINOCCHIO_UNIT_TEST(multiprecision-mpfr)
-      SET_PROPERTY(TARGET test-cpp-multiprecision-mpfr PROPERTY CXX_STANDARD 11)
-      TARGET_LINK_LIBRARIES(test-cpp-multiprecision-mpfr PUBLIC ${MPFR_LIBRARIES})
-      TARGET_INCLUDE_DIRECTORIES(test-cpp-multiprecision-mpfr PUBLIC ${MPFR_INCLUDE_DIRS})
-      TARGET_COMPILE_OPTIONS(test-cpp-multiprecision-mpfr PUBLIC ${MPFR_CFLAGS_OTHER})
-    ENDIF(MPFR_FOUND)
-  ENDIF(PkgConfig_FOUND)
+  ADD_COMPILE_DEPENDENCY(gmp)
+  ADD_COMPILE_DEPENDENCY(mpfr)
+
+  IF(MPFR_FOUND)
+    ADD_PINOCCHIO_UNIT_TEST(multiprecision-mpfr)
+    PKG_CONFIG_USE_DEPENDENCY(test-cpp-multiprecision-mpfr gmp)
+    PKG_CONFIG_USE_DEPENDENCY(test-cpp-multiprecision-mpfr mpfr)
+    SET_PROPERTY(TARGET test-cpp-multiprecision-mpfr PROPERTY CXX_STANDARD 11)
+  ENDIF(MPFR_FOUND)
 ENDIF(BUILD_ADVANCED_TESTING)
 
 # Automatic differentiation