cmake: fix static linking for bundled deps

Our bundled deps are being built as simple static libraries which are
then linked into the libgit2 library via `TARGET_LINK_LIBRARIES`. While
this works for a dynamically built libgit2 library, using this function
to link two static libraries does not have the expected outcome of
merging those static libraries into one big library. This leads to
symbols of our bundled deps being undefined in the resulting libgit2
archive.

As we have bumped our minimum CMake version to 2.8.11, we can now easily
make use of object libraries for our bundled dependencies. So build
instructions are still self-contained inside of the dependency
directories and the resulting object libraries can just be added to the
LIBGIT2_OBJECTS list, which will cause them to be linked into the final
resulting static library. This fixes the issue of undefined symbols.

deps/http-parser/CMakeLists.txt

 FILE(GLOB SRC_HTTP "*.c" "*.h")
 
-ADD_LIBRARY(http-parser STATIC ${SRC_HTTP})
+ADD_LIBRARY(http-parser OBJECT ${SRC_HTTP})

deps/regex/CMakeLists.txt

 INCLUDE_DIRECTORIES(".")
-ADD_LIBRARY(regex STATIC "regex.c" "regex.h")
+ADD_LIBRARY(regex OBJECT "regex.c" "regex.h")

deps/zlib/CMakeLists.txt

 ADD_DEFINITIONS(-DNO_VIZ -DSTDC -DNO_GZIP)
 FILE(GLOB SRC_ZLIB "*.c" "*.h")
 INCLUDE_DIRECTORIES(".")
-ADD_LIBRARY(zlib STATIC ${SRC_ZLIB})
+ADD_LIBRARY(zlib OBJECT ${SRC_ZLIB})

src/CMakeLists.txt

@@ -183,7 +183,7 @@ ENDIF()
 IF(WIN32 OR AMIGA OR CMAKE_SYSTEM_NAME MATCHES "(Solaris|SunOS)")
 	ADD_SUBDIRECTORY("${CMAKE_SOURCE_DIR}/deps/regex" "${CMAKE_BINARY_DIR}/deps/regex")
 	LIST(APPEND LIBGIT2_INCLUDES "${CMAKE_SOURCE_DIR}/deps/regex")
-	LIST(APPEND LIBGIT2_LIBS regex)
+	LIST(APPEND LIBGIT2_OBJECTS $<TARGET_OBJECTS:regex>)
 ENDIF()
 
 # Optional external dependency: http-parser
@@ -196,7 +196,7 @@ ELSE()
 	MESSAGE(STATUS "http-parser version 2 was not found or disabled; using bundled 3rd-party sources.")
 	ADD_SUBDIRECTORY("${CMAKE_SOURCE_DIR}/deps/http-parser" "${CMAKE_BINARY_DIR}/deps/http-parser")
 	LIST(APPEND LIBGIT2_INCLUDES "${CMAKE_SOURCE_DIR}/deps/http-parser")
-	LIST(APPEND LIBGIT2_LIBS http-parser)
+	LIST(APPEND LIBGIT2_OBJECTS "$<TARGET_OBJECTS:http-parser>")
 ENDIF()
 
 # Optional external dependency: zlib
@@ -214,7 +214,7 @@ ELSE()
 	MESSAGE(STATUS "zlib was not found; using bundled 3rd-party sources." )
 	ADD_SUBDIRECTORY("${CMAKE_SOURCE_DIR}/deps/zlib" "${CMAKE_BINARY_DIR}/deps/zlib")
 	LIST(APPEND LIBGIT2_INCLUDES "${CMAKE_SOURCE_DIR}/deps/zlib")
-	LIST(APPEND LIBGIT2_LIBS zlib)
+	LIST(APPEND LIBGIT2_OBJECTS $<TARGET_OBJECTS:zlib>)
 ENDIF()
 
 # Optional external dependency: libssh2