diff --git a/Externals/bzip2/CMakeLists.txt b/Externals/bzip2/CMakeLists.txt
index 5ab09692df..62e0e075c4 100644
--- a/Externals/bzip2/CMakeLists.txt
+++ b/Externals/bzip2/CMakeLists.txt
@@ -3,7 +3,6 @@ project(bzip2 C)
 include(CheckTypeSize)
 include(CheckFunctionExists)
 include(CheckIncludeFile)
-include(CheckCSourceCompiles)
 
 check_include_file(sys/types.h HAVE_SYS_TYPES_H)
 check_include_file(stdint.h    HAVE_STDINT_H)
diff --git a/Externals/liblzma/CMakeLists.txt b/Externals/liblzma/CMakeLists.txt
index 3c728a007e..0d33d0ad9d 100644
--- a/Externals/liblzma/CMakeLists.txt
+++ b/Externals/liblzma/CMakeLists.txt
@@ -3,7 +3,6 @@ project(lzma C)
 include(CheckTypeSize)
 include(CheckFunctionExists)
 include(CheckIncludeFile)
-include(CheckCSourceCompiles)
 
 check_include_file(sys/types.h HAVE_SYS_TYPES_H)
 check_include_file(stdint.h    HAVE_STDINT_H)
diff --git a/Externals/mbedtls/CMakeLists.txt b/Externals/mbedtls/CMakeLists.txt
index da206c0810..7941406d61 100644
--- a/Externals/mbedtls/CMakeLists.txt
+++ b/Externals/mbedtls/CMakeLists.txt
@@ -172,8 +172,6 @@ endfunction(link_to_source)
 
 string(REGEX MATCH "Clang" CMAKE_COMPILER_IS_CLANG "${CMAKE_C_COMPILER_ID}")
 
-include(CheckCCompilerFlag)
-
 if(CMAKE_COMPILER_IS_GNU)
     # some warnings we want are not available with old GCC versions
     # note: starting with CMake 2.8 we could use CMAKE_C_COMPILER_VERSION
diff --git a/Externals/zstd/CMakeLists.txt b/Externals/zstd/CMakeLists.txt
index fdcabb0956..9eb29e0037 100644
--- a/Externals/zstd/CMakeLists.txt
+++ b/Externals/zstd/CMakeLists.txt
@@ -3,7 +3,6 @@ project(zstd C)
 include(CheckTypeSize)
 include(CheckFunctionExists)
 include(CheckIncludeFile)
-include(CheckCSourceCompiles)
 
 check_include_file(sys/types.h HAVE_SYS_TYPES_H)
 check_include_file(stdint.h    HAVE_STDINT_H)