Merge bitcoin-core/secp256k1#1069: build: Replace use of deprecated a…

…utoconf macro AC_PROG_CC_C89

e0db3f8 build: Replace use of deprecated autoconf macro AC_PROG_CC_C89 (laanwj)

Pull request description:

ACKs for top commit:
  real-or-random:
    utACK e0db3f8
  jonasnick:
    ACK e0db3f8

Tree-SHA512: 00d6719fcdea69d002c795bbed07ccbd69900fef7dcba8ee42aa4e77765034feeb036ac9147b7fccc88b41623f735f62d4c72e25b3a1e68caad08a1237d6c5f5

configure.ac

@@ -38,8 +38,7 @@ AC_PATH_TOOL(AR, ar)
 AC_PATH_TOOL(RANLIB, ranlib)
 AC_PATH_TOOL(STRIP, strip)
 
-AM_PROG_CC_C_O
-AC_PROG_CC_C89
+AC_PROG_CC
 if test x"$ac_cv_prog_cc_c89" = x"no"; then
   AC_MSG_ERROR([c89 compiler support required])
 fi