cmake: fix optional dependency search

unittest/CMakeLists.txt

@@ -32,6 +32,11 @@ MACRO(ADD_PINOCCHIO_UNIT_TEST NAME)
   TARGET_LINK_LIBRARIES(${TEST_NAME} PRIVATE ${Boost_UNIT_TEST_FRAMEWORK_LIBRARY})
 ENDMACRO(ADD_PINOCCHIO_UNIT_TEST)
 
+MACRO(ADD_OPTIONAL_COMPILE_DEPENDENCY PKG_CONFIG_STRING)
+  _GET_PKG_CONFIG_DEBUG_STRING("${PKG_CONFIG_STRING}" ${ARGN})
+  ADD_DEPENDENCY(0 1 ${PKG_CONFIG_STRING} "${PKG_CONFIG_DEBUG_STRING}")
+ENDMACRO()
+
 # --- RULES -------------------------------------------------------------------
 # --- RULES -------------------------------------------------------------------
 # --- RULES -------------------------------------------------------------------
@@ -136,8 +141,8 @@ IF(BUILD_ADVANCED_TESTING)
   ADD_PINOCCHIO_UNIT_TEST(multiprecision)
   SET_PROPERTY(TARGET test-cpp-multiprecision PROPERTY CXX_STANDARD 11)
 
-  ADD_COMPILE_DEPENDENCY(gmp)
-  ADD_COMPILE_DEPENDENCY(mpfr)
+  ADD_OPTIONAL_COMPILE_DEPENDENCY(gmp)
+  ADD_OPTIONAL_COMPILE_DEPENDENCY(mpfr)
 
   IF(MPFR_FOUND)
     ADD_PINOCCHIO_UNIT_TEST(multiprecision-mpfr)