[cmake] Fix handling of BUILD_SHARED_LIBS option

BUILD_SHARED_LIBS option is declared conditionally on whether
BUILD_SHARED_LIBS is defined itself. This is problematic as multiple
invocations of a build runner such as ninja will cause builds to use
different options. The first build will run as if BUILD_SHARED_LIBS is
not defined (i.e. OFF option), the second time cmake will notice that
BUILD_SHARED_LIBS was defined after all and now run build with
BUILD_SHARED_LIBS set to ON. This is confusing and error prone.

To fix the problem, a top-level BUILD_SHARED_LIBS option has been added
and BUILD_SHARED_LIBS option in applications subdirectory was changed to
become active only if the project is standalone.

Author: p12tic

CMakeLists.txt

@@ -28,6 +28,7 @@ option(CCTAG_ENABLE_SIMD_AVX2 "Enable AVX2 optimizations" OFF)
 option(CCTAG_BUILD_TESTS "Build the unity tests" ON)
 option(CCTAG_BUILD_DOC "Build documentation" OFF)
 option(CCTAG_NO_THRUST_COPY_IF "Do not use thrust::copy_if() on GPU. There may be a bug on CUDA 7 with GTX 980, 980Ti and 1080" OFF)
+option(BUILD_SHARED_LIBS "Build shared libraries" ON)
 
 set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${PROJECT_SOURCE_DIR}/cmake")
 

src/applications/CMakeLists.txt

@@ -8,10 +8,8 @@ if(NOT CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR)
 
   set(CMAKE_CXX_STANDARD 14)
   set(CMAKE_CXX_STANDARD_REQUIRED ON)
-endif()
 
-# if this is used as a stand-alone project we need to tell whether to use PIC
-if(NOT DEFINED BUILD_SHARED_LIBS)
+  # if this is used as a stand-alone project we need to tell whether to use PIC
   option(BUILD_SHARED_LIBS "Build shared libraries" ON)
   set(CMAKE_POSITION_INDEPENDENT_CODE ${BUILD_SHARED_LIBS})
 endif()