tests: fix leading whitespace in linker flags

Building a project as ExternalProject in Tarantool is failed
because initial LDFLAGS is empty and after adding an option
a final value has a leading whitespace, but should not [1]:

CMake Error at tests/capi/CMakeLists.txt:69 (add_executable):
  Target "ffi_cdef_proto_test" links to item " -g" which has leading or
  trailing whitespace.  This is now an error according to policy CMP0004.
Call Stack (most recent call first):
  tests/capi/ffi_cdef_proto/CMakeLists.txt:16 (create_test)

The patch add a macro `AppendFlags` that allow to avoid such
issues.

1. https://cmake.org/cmake/help/latest/policy/CMP0004.html

Author: ligurio

cmake/utils.cmake

@@ -32,3 +32,13 @@ function(lua_source varname filename symbolname)
     set(var ${${varname}})
     set(${varname} ${var} ${dstfile} PARENT_SCOPE)
 endfunction()
+
+macro(AppendFlags flags)
+    foreach(flag ${ARGN})
+        if (${flags})
+            set(${flags} "${${flags}} ${flag}")
+        else()
+            set(${flags} "${flag}")
+        endif()
+    endforeach()
+endmacro()

tests/capi/CMakeLists.txt

@@ -27,20 +27,29 @@ target_link_libraries(
         >
 )
 
+# The following condition looks unnecessary, it was added to fix
+# an issue for Sydr: in the Lua external project in the build
+# system the explicit build command is used:
+# BUILD_COMMAND cd <SOURCE_DIR> && make -j CC=${CMAKE_C_COMPILER} CFLAGS=${CFLAGS} LDFLAGS=${LDFLAGS},
+# which use only flags for Lua build, which were explicitly
+# declared earlier, so we need to add `-g` to CFLAGS by ourselves.
+# See [1].
+#
+# 1. https://github.com/ligurio/lua-c-api-tests/pull/6#discussion_r1185003511
 if (CMAKE_BUILD_TYPE STREQUAL "Debug")
-  set(LDFLAGS "${LDFLAGS} ${CMAKE_C_FLAGS_DEBUG}")
+  AppendFlags(LDFLAGS ${CMAKE_C_FLAGS_DEBUG})
 endif()
 
 if (ENABLE_ASAN)
-  set(LDFLAGS "${LDFLAGS} -fsanitize=address")
+  AppendFlags(LDFLAGS -fsanitize=address)
 endif()
 
 if (ENABLE_UBSAN)
-  set(LDFLAGS "${LDFLAGS} -fsanitize=undefined")
+  AppendFlags(LDFLAGS -fsanitize=undefined)
 endif()
 
 if (ENABLE_COV)
-  set(LDFLAGS "${LDFLAGS} -fprofile-instr-generate -fcoverage-mapping")
+  AppendFlags(LDFLAGS -fprofile-instr-generate -fcoverage-mapping)
 endif()
 
 set(DEFAULT_RUNS_NUMBER 5)