Updated steps for preempting the target Thread::Thread in h5ppConfig.…

…cmake.in

* Some builds of HDF5 can link directly to -lpthread causing problems downstream.

cmake/h5ppConfig.cmake.in

@@ -9,32 +9,22 @@ list(INSERT CMAKE_MODULE_PATH 0 "${CMAKE_CURRENT_LIST_DIR}")
 include(CMakeFindDependencyMacro)
 
 
-########################## IMPORTANT #############################
+##################################################################
 ### Preempt Threads::Threads                                   ###
-### It's looked for in dependency configs, so we make it right ###
+### It's looked for in dependencies, so we make it right       ###
 ### before it's done wrong, i.e. with pthread instead of       ###
-### -lpthread. If this is undesirable you can preempt it       ###
-### yourself similarly. The reason is that downstream I often  ###
-### need to avoid a static linking segfault by using:          ###
-### -Wl, -whole-archive pthread -Wl, -no-whole-archive         ###
-### and cmake is unable to identify "-lpthread" as being the   ###
-### same as pthread --> multiple declaration errors. Note that ###
-### CMake changes it into                                      ###
-### -Wl, -whole-archive -lpthread -Wl, -no-whole-archive       ###
-### anyway. If an entire dependency tree had only "-lpthread"  ###
-### or only "pthread" this wouldn't be a problem, but          ###
-### unfortunately different dependencies do it differently.    ###
-### Since conan prefers "pthread", we do the same here.        ###
+### -lpthread.                                                 ###
+### Otherwise, we get linker errors consuming h5pp             ###
+### Here we specify the linking twice                          ###
+### 1) As string to make sure -lpthread gets sandwiched by     ###
+###    -Wl,--whole-archive.... -Wl,--no-whole-archive          ###
+### 2) As usual to make sure that if somebody links            ###
+###    Threads::Threads, then any repeated pthread appended    ###
+###    to the end (the wrong order causes linking errors)      ###
 ##################################################################
-### Read more about pthread segfault
-### https://stackoverflow.com/questions/35116327/when-g-static-link-pthread-cause-segmentation-fault-why
 if(NOT TARGET Threads::Threads)
-    set(CMAKE_THREAD_PREFER_PTHREAD FALSE)
-    set(THREADS_PREFER_PTHREAD_FLAG FALSE)
+    set(THREADS_PREFER_PTHREAD_FLAG TRUE)
     find_package(Threads)
-    if(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
-        set_target_properties(Threads::Threads PROPERTIES INTERFACE_LINK_LIBRARIES pthread)
-    endif()
 endif()