Update build logic and prevent conflicting options

cmake/HandleGeneralOptions.cmake

@@ -47,7 +47,9 @@ option(GTSAM_TANGENT_PREINTEGRATION         "Use new ImuFactor with integration
 option(GTSAM_SLOW_BUT_CORRECT_BETWEENFACTOR "Use the slower but correct version of BetweenFactor" OFF)
 option(GTSAM_SLOW_BUT_CORRECT_EXPMAP        "Use slower but correct expmap for Pose2"  OFF)
 
-if (GTSAM_FORCE_SHARED_LIB)
+if (GTSAM_FORCE_SHARED_LIB AND GTSAM_FORCE_STATIC_LIB)
+    message(FATAL_ERROR "GTSAM_FORCE_SHARED_LIB and GTSAM_FORCE_STATIC_LIB are both true. Please, to unambiguously select the desired library type to use to build GTSAM, set one of GTSAM_FORCE_SHARED_LIB=ON, GTSAM_FORCE_STATIC_LIB=ON, or BUILD_SHARED_LIBS={ON/OFF}")
+elseif (GTSAM_FORCE_SHARED_LIB)
     message(STATUS "GTSAM is a shared library due to GTSAM_FORCE_SHARED_LIB")
     set(GTSAM_LIBRARY_TYPE SHARED CACHE STRING "" FORCE)
     set(GTSAM_SHARED_LIB 1 CACHE BOOL "" FORCE)
@@ -56,10 +58,9 @@ elseif (GTSAM_FORCE_STATIC_LIB)
     set(GTSAM_LIBRARY_TYPE STATIC CACHE STRING "" FORCE)
     set(GTSAM_SHARED_LIB 0 CACHE BOOL "" FORCE)
 elseif (BUILD_SHARED_LIBS)
-    message(STATUS "GTSAM is a shared library due to BUILD_SHARED_LIBS is ON")
     set(GTSAM_LIBRARY_TYPE SHARED CACHE STRING "" FORCE)
     set(GTSAM_SHARED_LIB 1 CACHE BOOL "" FORCE)
-elseif((DEFINED BUILD_SHARED_LIBS) AND (NOT BUILD_SHARED_LIBS))
+elseif(NOT BUILD_SHARED_LIBS)
     message(STATUS "GTSAM is a static library due to BUILD_SHARED_LIBS is OFF")
     set(GTSAM_LIBRARY_TYPE STATIC CACHE STRING "" FORCE)
     set(GTSAM_SHARED_LIB 0 CACHE BOOL "" FORCE)