Fix compilation using default options on Emscripten (#470)

* Fix compilation using default options on Emscripten

By default, fcl compiles with the `FCL_USE_X64_SSE` option enabled, that compiles the library 
with SSE flags enabled. This creates problems in any compiler that do not targets x86 or x86_64 . 

While the problem can be easily solved by user by setting the FCL_USE_X64_SSE to OFF 
(as done for example in Debian packaging scripts, see https://salsa.debian.org/science-team/fcl/-/blob/61641fb7a12d19806c799503fc90fcd64b20c5ad/debian/rules#L10)  it is nice to provide a working 
build out of the box.

CMakeLists.txt

@@ -86,8 +86,13 @@ else()
 endif()
 
 # Whether to enable SSE
+if(CMAKE_SYSTEM_NAME STREQUAL "Emscripten")
+  set(FCL_TARGET_SUPPORT_X64_SSE OFF)
+else()
+  set(FCL_TARGET_SUPPORT_X64_SSE ON)
+endif()
 set(SSE_FLAGS "")
-option(FCL_USE_X64_SSE "Whether FCL should x64 SSE instructions" ON)
+option(FCL_USE_X64_SSE "Whether FCL should x64 SSE instructions" ${FCL_TARGET_SUPPORT_X64_SSE})
 if(FCL_USE_X64_SSE)
   if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
     set(SSE_FLAGS -mfpmath=sse -msse -msse2 -msse3 -mssse3)