Use CMAKE_PREFIX_PATH to look for locally installed IpOpt

cmake/modules/FindIPOpt.cmake

@@ -8,20 +8,20 @@ find_library(DL_LIBRARY dl)
 
 find_path(IPOPT_INCLUDE_DIR
   NAMES "IpNLP.hpp"
-  HINTS ${IPOPT_PREFIX} ${IPOPT_ROOT}
+  HINTS ${CMAKE_PREFIX_PATH}
   PATH_SUFFIXES "include" "include/coin"
 )
 
 find_library(IPOPT_LIBRARY
   NAMES ipopt
-  HINTS ${IPOPT_PREFIX} ${IPOPT_ROOT}
+  HINTS ${CMAKE_PREFIX_PATH}
   PATH_SUFFIXES "lib"
 )
 
 # If you want to want to use other linear solver
 find_library(HSL_LIBRARY
   NAMES coinhsl
-  HINTS ${IPOPT_PREFIX} ${IPOPT_ROOT}
+  HINTS ${CMAKE_PREFIX_PATH}
   PATH_SUFFIXES "lib"
 )