remove POSIX_C_SOURCE and remove dlfcn.h for static builds

The former was a faulty "fix" for gmtime_r not existing on Windows. The latter is needed only for dynamic builds, and is not included with msys2, which ends up fine because Windows is only built static at this time.

CMakeLists.txt

@@ -323,7 +323,7 @@ else()
   endif()
   if(MINGW)
     set(WARNINGS "${WARNINGS} -Wno-error=unused-value -Wno-error=unused-but-set-variable")
-    set(MINGW_FLAG "${MINGW_FLAG} -DWIN32_LEAN_AND_MEAN -D_POSIX_C_SOURCE")
+    set(MINGW_FLAG "${MINGW_FLAG} -DWIN32_LEAN_AND_MEAN")
     set(Boost_THREADAPI win32)
     include_directories(SYSTEM src/platform/mingw)
     # mingw doesn't support LTO (multiple definition errors at link time)

src/common/stack_trace.cpp

@@ -33,7 +33,9 @@
 #include <libunwind.h>
 #endif
 #include <cxxabi.h>
+#ifndef STATICLIB
 #include <dlfcn.h>
+#endif
 
 // from http://stackoverflow.com/questions/11665829/how-can-i-print-stack-trace-for-caught-exceptions-in-c-code-injection-in-c
 #ifdef STATICLIB