[cmake] only require a CXX compiler when tests are build

Signed-off-by: Thomas Devoogdt <thomas@devoogdt.com>

build/cmake/CMakeLists.txt

@@ -37,7 +37,6 @@ project(zstd
   VERSION "${ZSTD_FULL_VERSION}"
   LANGUAGES C   # Main library is in C
             ASM # And ASM
-            CXX # Testing contributed code also utilizes CXX
   )
 
 message(STATUS "ZSTD VERSION: ${zstd_VERSION}")
@@ -54,12 +53,6 @@ endif()
 
 include(GNUInstallDirs)
 
-#-----------------------------------------------------------------------------
-# Add extra compilation flags
-#-----------------------------------------------------------------------------
-include(AddZstdCompilationFlags)
-ADD_ZSTD_COMPILATION_FLAGS()
-
 # Always hide XXHash symbols
 add_definitions(-DXXH_NAMESPACE=ZSTD_)
 
@@ -123,6 +116,19 @@ if (MSVC)
     option(ZSTD_USE_STATIC_RUNTIME "LINK TO STATIC RUN-TIME LIBRARIES" OFF)
 endif ()
 
+# Enable C++ support for testing.
+set(ZSTD_ENABLE_CXX ${ZSTD_BUILD_TESTS})
+
+if(ZSTD_ENABLE_CXX)
+    enable_language(CXX)
+endif()
+
+#-----------------------------------------------------------------------------
+# Add extra compilation flags
+#-----------------------------------------------------------------------------
+include(AddZstdCompilationFlags)
+ADD_ZSTD_COMPILATION_FLAGS(ON ZSTD_ENABLE_CXX ON) # C CXX LD
+
 #-----------------------------------------------------------------------------
 # External dependencies
 #-----------------------------------------------------------------------------