diff --git a/lib/base64.c b/lib/base64.c
index 30db7e5889..2a49b5acd6 100644
--- a/lib/base64.c
+++ b/lib/base64.c
@@ -31,7 +31,7 @@
   !defined(CURL_DISABLE_SMTP) || \
   !defined(CURL_DISABLE_POP3) || \
   !defined(CURL_DISABLE_IMAP) || \
-  !defined(CURL_DISABLE_DOH) || defined(USE_SSL)
+  !defined(CURL_DISABLE_DOH) || defined(USE_SSL) || defined(BUILDING_CURL)
 #include "curl/curl.h"
 #include "warnless.h"
 #include "curl_base64.h"
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index f6517c7036..a3c4218eaf 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -22,6 +22,7 @@
 #
 ###########################################################################
 set(EXE_NAME curl)
+add_definitions(-DBUILDING_CURL)
 
 if(USE_MANUAL)
   # Use the C locale to ensure that only ASCII characters appear in the
diff --git a/src/Makefile.am b/src/Makefile.am
index ed2df5ef57..ddeb70073a 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -50,6 +50,7 @@ SUBDIRS = ../docs
 if USE_CPPFLAG_CURL_STATICLIB
 AM_CPPFLAGS += -DCURL_STATICLIB
 endif
+AM_CPPFLAGS += -DBUILDING_CURL
 
 include Makefile.inc