only look for MPFR if boost is available

CHANGELOG

@@ -35,6 +35,7 @@ fixed bugs:
 - fix bugs in function definitions in soplex_interface.cpp: SoPlex_changeVarUpperReal(), SoPlex_getUpperReal()
 - fix bugs in LP file reader, concerning rational numbers as well as dynamic line length
 - fix bug printing solutions and statistics when running from the command line
+- fix bug in cmake build system when MPFR is available but boost is not
 
 Upcoming Bugfix Release 6.0.4
 =============================

CMakeLists.txt

@@ -141,19 +141,6 @@ if(GMP_FOUND)
     set(libs ${libs} ${GMP_LIBRARIES})
 endif()
 
-if(MPFR)
-  find_package(MPFR)
-endif()
-if(MPFR_FOUND)
-    message("SoPlex with Boost MPFR libraries.")
-    set(SOPLEX_WITH_MPFR on)
-    include_directories(${MPFR_INCLUDE_DIRS})
-    set(libs ${libs} ${MPFR_LIBRARIES})
-else()
-    message("-- SoPlex with Boost CPP multiprecision libraries.")
-    set(SOPLEX_WITH_CPPMPF on)
-endif()
-
 if(QUADMATH)
    find_package(Quadmath)
 endif()
@@ -196,6 +183,18 @@ if(BOOST)
                        Found Boost version is ${Boost_VERSION_STRING}")
             endif()
         endif()
+        if(MPFR) # MPFR is used within boost multiprecision, so using it without Boost does not make sense
+            find_package(MPFR)
+        endif()
+        if(MPFR_FOUND)
+            message("SoPlex with Boost MPFR libraries.")
+            set(SOPLEX_WITH_MPFR on)
+            include_directories(${MPFR_INCLUDE_DIRS})
+            set(libs ${libs} ${MPFR_LIBRARIES})
+        else()
+            message("-- SoPlex with Boost CPP multiprecision libraries.")
+            set(SOPLEX_WITH_CPPMPF on)
+        endif()
     else()
         set(BOOST off)
     endif()