build: Fix warnings in x86_64 assembly check

This change fixes:
- `-Wuninitialized` in both Autotools and CMake;
- `-Wreturn-type` in CMake only.

build-aux/m4/bitcoin_secp.m4

@@ -3,7 +3,7 @@ AC_DEFUN([SECP_X86_64_ASM_CHECK],[
 AC_MSG_CHECKING(for x86_64 assembly availability)
 AC_LINK_IFELSE([AC_LANG_PROGRAM([[
   #include <stdint.h>]],[[
-  uint64_t a = 11, tmp;
+  uint64_t a = 11, tmp = 0;
   __asm__ __volatile__("movq \@S|@0x100000000,%1; mulq %%rsi" : "+a"(a) : "S"(tmp) : "cc", "%rdx");
   ]])], [has_x86_64_asm=yes], [has_x86_64_asm=no])
 AC_MSG_RESULT([$has_x86_64_asm])

cmake/CheckX86_64Assembly.cmake

@@ -4,10 +4,11 @@ function(check_x86_64_assembly)
   check_c_source_compiles("
     #include <stdint.h>
 
-    int main()
+    int main(void)
     {
-      uint64_t a = 11, tmp;
+      uint64_t a = 11, tmp = 0;
       __asm__ __volatile__(\"movq $0x100000000,%1; mulq %%rsi\" : \"+a\"(a) : \"S\"(tmp) : \"cc\", \"%rdx\");
+      return 0;
     }
   " HAVE_X86_64_ASM)
   set(HAVE_X86_64_ASM ${HAVE_X86_64_ASM} PARENT_SCOPE)