cmake: support BUILD_SHARED_LIBS built-in option

Support building internal libraries as shared. This reduces
development time by eliminating the need to re-link all
binaries every time non-interface code in the library changes.
Instead, can hack on libxyz, then `make libxyz`, and re-run
monerod.

By default BUILD_SHARED_LIBS is OFF in release build type,
and ON in debug build type, but can be overriden with -D.

contrib/otshell_utils/CMakeLists.txt

@@ -9,6 +9,6 @@ file(GLOB otshell_utils_sources # All files in directory:
 	"*.cpp"
 )
 
-add_library (otshell_utils STATIC ${otshell_utils_sources})
+add_library (otshell_utils ${otshell_utils_sources})
 set_target_properties (otshell_utils PROPERTIES OUTPUT_NAME "otshell_utils")
 #target_link_libraries (upnpc-static ${LDLIBS}) # to add used libs