[build][cmake] Fix cmake with custom assembler

Tell CMake to explicitly compile our assembly as C code, because we
require it is compiled by a C compiler, and it is only enabled for
clang/gcc.

Fixes #3193.

build/cmake/lib/CMakeLists.txt

@@ -77,6 +77,12 @@ if (MSVC)
     set(PlatformDependResources ${MSVC_RESOURCE_DIR}/libzstd-dll.rc)
 endif ()
 
+# Explicitly set the language to C for all files, including ASM files.
+# Our assembly expects to be compiled by a C compiler, and is only enabled for
+# __GNUC__ compatible compilers. Otherwise all the ASM code is disabled by
+# macros.
+set_source_files_properties(${Sources} PROPERTIES LANGUAGE C)
+
 # Split project to static and shared libraries build
 set(library_targets)
 if (ZSTD_BUILD_SHARED)