cmake/FindLDAP: avoid empty 'Requires' item when omitting `pkg-config` module

`list(APPEND ...)` does this automatically, but we're _prepending_ LDAP
(to follow historical code and also autotools). `list(PREPEND ...)`
would likely do that the same, but it requires CMake 3.15 so we do this
manually and the manual method needs to skip the empty value manually.

Follow-up to 49f2a23d509645d534cbb2e2ffbd6347fac6e59e #15273
Closes #15828

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3400327b50557a434782475eebda6acd445409f6..197e94a0eb1c103c7e9cce19ceb64035306774b3 100644 (file)
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1115,7 +1115,9 @@ if(NOT CURL_DISABLE_LDAP)
       set(HAVE_LBER_H 1)
       set(CURL_LIBS "${LDAP_LIBRARIES};${CURL_LIBS}")
       list(APPEND CURL_LIBDIRS ${LDAP_LIBRARY_DIRS})
-      set(LIBCURL_PC_REQUIRES_PRIVATE "${LDAP_PC_REQUIRES};${LIBCURL_PC_REQUIRES_PRIVATE}")
+      if(LDAP_PC_REQUIRES)
+        set(LIBCURL_PC_REQUIRES_PRIVATE "${LDAP_PC_REQUIRES};${LIBCURL_PC_REQUIRES_PRIVATE}")
+      endif()
       include_directories(SYSTEM ${LDAP_INCLUDE_DIRS})
       link_directories(${LDAP_LIBRARY_DIRS})
       if(LDAP_CFLAGS)