Rework the coverage build

This reworks it to avoid a need to special build type and adding the flags only
to the targets that need it (skipping testing wrappers which break with them).

It also updates the CodeCoverage module from the following URL:

https://github.com/bilke/cmake-modules/blob/master/CodeCoverage.cmake

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

tests/server/test_server/CMakeLists.txt

@@ -12,6 +12,9 @@ add_library(testserver STATIC
             test_server.c
             default_cb.c
             sftpserver_cb.c)
+if (WITH_COVERAGE)
+    append_coverage_compiler_flags_to_target(testserver)
+endif (WITH_COVERAGE)
 
 set(LIBSSH_SERVER_TESTS
 #    torture_server_kbdint
@@ -29,10 +32,10 @@ if (UNIX AND NOT WIN32)
     add_executable(test_server ${server_SRCS})
     target_compile_options(test_server PRIVATE ${DEFAULT_C_COMPILE_FLAGS})
     target_link_libraries(test_server
-                          testserver
-                          ssh::ssh
-                          ${ARGP_LIBRARIES}
-                          util)
+        PRIVATE testserver ssh::ssh ${ARGP_LIBRARIES} util)
+    if (WITH_COVERAGE)
+        append_coverage_compiler_flags_to_target(test_server)
+    endif (WITH_COVERAGE)
 endif ()
 
 endif (WITH_SERVER AND UNIX AND NOT WIN32)