cmake: merge libs via virtual object libraries

CMake issued a warming about policy CMP0026: access of LOCATION
target property at config time was disallowed. Offending code
was the code that merged static libraries to generate
libwallet_merged.a.

This patch does that same merge task in a much simpler way. And,
since it doesn't violate the policy, the warning went away.

src/CMakeLists.txt

@@ -82,8 +82,13 @@ function (bitmonero_add_library name)
     FILES
       ${ARGN})
 
-  add_library("${name}"
-    ${ARGN})
+  # Define a ("virtual") object library and an actual library that links those
+  # objects together. The virtual libraries can be arbitrarily combined to link
+  # any subset of objects into one library archive. This is used for releasing
+  # libwallet, which combines multiple components.
+  set(objlib obj_${name})
+  add_library(${objlib} OBJECT ${ARGN})
+  add_library("${name}" STATIC $<TARGET_OBJECTS:${objlib}>)
   set_property(TARGET "${name}"
     PROPERTY
       FOLDER "libs")

src/wallet/CMakeLists.txt

@@ -27,7 +27,6 @@
 # THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 # include (${PROJECT_SOURCE_DIR}/cmake/libutils.cmake)
-include (${PROJECT_SOURCE_DIR}/cmake/MergeStaticLibs.cmake)
 
 set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 
@@ -76,7 +75,11 @@ target_link_libraries(wallet
     ${EXTRA_LIBRARIES})
 
 set(libs_to_merge wallet cryptonote_core mnemonics common crypto)
-merge_static_libs(wallet_merged "${libs_to_merge}")
+
+foreach(lib ${libs_to_merge})
+    list(APPEND objlibs $<TARGET_OBJECTS:obj_${lib}>) # matches naming convention in src/CMakeLists.txtA
+endforeach()
+add_library(wallet_merged STATIC ${objlibs})
 install(TARGETS wallet_merged
     ARCHIVE DESTINATION lib)