Merge #373: build: fix x86_64 asm detection for some compilers

3f8fdfb build: fix x86_64 asm detection for some compilers (Cory Fields)

build-aux/m4/bitcoin_secp.m4

@@ -3,13 +3,13 @@ AC_DEFUN([SECP_INT128_CHECK],[
 has_int128=$ac_cv_type___int128
 ])
 
-dnl 
+dnl escape "$0x" below using the m4 quadrigaph @S|@, and escape it again with a \ for the shell.
 AC_DEFUN([SECP_64BIT_ASM_CHECK],[
 AC_MSG_CHECKING(for x86_64 assembly availability)
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
   #include <stdint.h>]],[[
   uint64_t a = 11, tmp;
-  __asm__ __volatile__("movq $0x100000000,%1; mulq %%rsi" : "+a"(a) : "S"(tmp) : "cc", "%rdx");
+  __asm__ __volatile__("movq \@S|@0x100000000,%1; mulq %%rsi" : "+a"(a) : "S"(tmp) : "cc", "%rdx");
   ]])],[has_64bit_asm=yes],[has_64bit_asm=no])
 AC_MSG_RESULT([$has_64bit_asm])
 ])