Require static boost libs only if necessary

Some distros don't ship static libraries by default, therefore requiring
them by default breaks on those. Also, it makes no sense, really.

Fixes one build issue on openSUSE Leap.

src/CMakeLists.txt

@@ -4,7 +4,10 @@ cmake_minimum_required(VERSION 3.5)
 # globally include own includes
 include_directories(${PROJECT_SOURCE_DIR}/include)
 
-set(Boost_USE_STATIC_LIBS ON)
+if(STATIC_BUILD)
+    set(Boost_USE_STATIC_LIBS ON)
+endif()
+
 find_package(Boost REQUIRED COMPONENTS filesystem regex)
 set(BOOST_LIBS Boost::filesystem Boost::regex)