build: remove assembly detection when explicitly disabled

Author: Raimo33

CMakeLists.txt

@@ -99,7 +99,7 @@ if(SECP256K1_ASM STREQUAL "arm32")
   else()
     message(FATAL_ERROR "ARM32 assembly requested but not available.")
   endif()
-elseif(SECP256K1_ASM)
+elseif(NOT SECP256K1_ASM STREQUAL "OFF")
   include(CheckX86_64Assembly)
   check_x86_64_assembly()
   if(HAVE_X86_64_ASM)