cmake: hide POSIX support to ENABLE_LAPI_TESTS

Follows up commit 0508724 ("tests/lapi: add string tests").

Author: ligurio

cmake/BuildLua.cmake

@@ -62,12 +62,11 @@ macro(build_lua LUA_VERSION)
         # with -fPIC".
         set(CFLAGS "${CFLAGS} -fPIC")
         set(CFLAGS "${CFLAGS} -DLUA_USE_DLOPEN")
+        # `io.popen()` is not supported by default, it is enabled
+        # by `LUA_USE_POSIX` flag. Required by a function `random_locale()`.
+        set(CFLAGS "${CFLAGS} -DLUA_USE_POSIX")
     endif()
 
-    # `io.popen()` is not supported by default, it is enabled
-    # by `LUA_USE_POSIX` flag. Required by a function `random_locale()`.
-    set(CFLAGS "${CFLAGS} -DLUA_USE_POSIX")
-
     include(ExternalProject)
 
     set(LUA_LIBRARY ${PROJECT_BINARY_DIR}/lua-${LUA_VERSION}/source/liblua.a)