diff --git a/CMakeLists.txt b/CMakeLists.txt
index 536b92c9..58e4d184 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,10 +1,11 @@
 cmake_minimum_required (VERSION 3.1)
+project (opendht)
+
 include(CMakePackageConfigHelpers)
 include(CMakeDependentOption)
 include(FindPkgConfig)
 include(cmake/CheckAtomic.cmake)
 
-project (opendht)
 set (opendht_VERSION_MAJOR 2)
 set (opendht_VERSION_MINOR 1.7)
 set (opendht_VERSION ${opendht_VERSION_MAJOR}.${opendht_VERSION_MINOR})
@@ -301,9 +302,11 @@ if (OPENDHT_HTTP)
   list (APPEND opendht_HEADERS
     include/opendht/proxy.h
     include/opendht/http.h
+    src/compat/os_cert.h
   )
   list (APPEND opendht_SOURCES
     src/http.cpp
+    src/compat/os_cert.cpp
   )
 endif ()
 
@@ -324,13 +327,8 @@ if(OPENDHT_ARGON2)
     include_directories(argon2/include/)
 endif()
 
-# cross-platform compatibility sources
-set (COMPAT_DIR ${PROJECT_SOURCE_DIR}/src/compat/)
-list (APPEND opendht_SOURCES ${COMPAT_DIR}/os_cert.cpp)
-
 if (MSVC)
-    set (MSC_COMPAT_DIR ${COMPAT_DIR}/msvc/)
-    list (APPEND opendht_SOURCES ${MSC_COMPAT_DIR}/unistd.h)
+    list (APPEND opendht_HEADERS src/compat/msvc/unistd.h)
 endif ()
 
 # Targets
diff --git a/src/compat/os_cert.cpp b/src/compat/os_cert.cpp
index ea113e08..7e80c5b8 100644
--- a/src/compat/os_cert.cpp
+++ b/src/compat/os_cert.cpp
@@ -39,6 +39,7 @@
 #define GENTIME_LENGTH 15
 #define UTCTIME_LENGTH 13
 #define ATOI2(ar)      ((ar) += 2, ((ar)[-2] - '0') * 10 + ((ar)[-1] - '0'))
+extern "C" {
 // Taken from libressl/src/crypto/asn1/a_time_tm.c
 int
 ASN1_time_parse(const char* bytes, size_t len, struct tm* tm, int mode)
@@ -116,7 +117,8 @@ ASN1_time_parse(const char* bytes, size_t len, struct tm* tm, int mode)
         return (-1);
     }
 
-    return (type);
+    return type;
+}
 }
 #endif /* EMBEDDED_ASN1_TIME_PARSE */
 
diff --git a/src/compat/os_cert.h b/src/compat/os_cert.h
index 25686543..b037037d 100644
--- a/src/compat/os_cert.h
+++ b/src/compat/os_cert.h
@@ -34,6 +34,7 @@
 #define V_ASN1_UTCTIME         23
 #define V_ASN1_GENERALIZEDTIME 24
 
+extern "C" {
 /*
  * Parse an RFC 5280 format ASN.1 time string.
  *
@@ -50,6 +51,7 @@
  * Fills in *tm with the corresponding time if tm is non NULL.
  */
 int ASN1_time_parse(const char* bytes, size_t len, struct tm* tm, int mode);
+}
 #endif
 
 namespace dht {