[build][cmake] Fix cmake with custom assembler

Fixes cmake build when the C compiler isn't used as the assembler.
We need to explicitly set the flag `-x assembler-with-cpp` to enable
the preprocessor.

Fixes facebook#3193.

build/cmake/CMakeLists.txt

@@ -40,11 +40,13 @@ if( CMAKE_MAJOR_VERSION LESS 3 )
   set(PROJECT_VERSION_PATCH ${zstd_VERSION_PATCH})
   set(PROJECT_VERSION "${zstd_VERSION_MAJOR}.${zstd_VERSION_MINOR}.${zstd_VERSION_PATCH}")
   enable_language(C)   # Main library is in C
+  enable_language(ASM) # And ASM
   enable_language(CXX) # Testing contributed code also utilizes CXX
 else()
   project(zstd
     VERSION "${zstd_VERSION_MAJOR}.${zstd_VERSION_MINOR}.${zstd_VERSION_PATCH}"
     LANGUAGES C   # Main library is in C
+              ASM # And ASM
               CXX # Testing contributed code also utilizes CXX
     )
 endif()
@@ -67,6 +69,7 @@ include(GNUInstallDirs)
 #-----------------------------------------------------------------------------
 include(AddZstdCompilationFlags)
 ADD_ZSTD_COMPILATION_FLAGS()
+SET(CMAKE_ASM_FLAGS "${CMAKE_ASM_FLAGS} -x assembler-with-cpp")
 
 # Always hide XXHash symbols
 add_definitions(-DXXH_NAMESPACE=ZSTD_)