Convert the rest of the codebase to C89.

Update build system to enforce -std=c89 -pedantic.

configure.ac

@@ -5,7 +5,7 @@ AC_CONFIG_MACRO_DIR([build-aux/m4])
 AC_CANONICAL_HOST
 AH_TOP([#ifndef LIBSECP256K1_CONFIG_H])
 AH_TOP([#define LIBSECP256K1_CONFIG_H])
-AH_BOTTOM([#endif //LIBSECP256K1_CONFIG_H])
+AH_BOTTOM([#endif /*LIBSECP256K1_CONFIG_H*/])
 AM_INIT_AUTOMAKE([foreign subdir-objects])
 LT_INIT
 
@@ -22,9 +22,9 @@ if test "x$CFLAGS" = "x"; then
   CFLAGS="-O3 -g"
 fi
 
-AC_PROG_CC_C99
-if test x"$ac_cv_prog_cc_c99" = x"no"; then
-  AC_MSG_ERROR([c99 compiler support required])
+AC_PROG_CC_C89
+if test x"$ac_cv_prog_cc_c89" = x"no"; then
+  AC_MSG_ERROR([c89 compiler support required])
 fi
 
 case $host in
@@ -70,7 +70,7 @@ esac
 
 CFLAGS="$CFLAGS -W"
 
-warn_CFLAGS="-Wall -Wextra -Wcast-align -Wnested-externs -Wshadow -Wstrict-prototypes -Wno-unused-function"
+warn_CFLAGS="-std=c89 -pedantic -Wall -Wextra -Wcast-align -Wnested-externs -Wshadow -Wstrict-prototypes -Wno-unused-function -Wno-long-long"
 saved_CFLAGS="$CFLAGS"
 CFLAGS="$CFLAGS $warn_CFLAGS"
 AC_MSG_CHECKING([if ${CC} supports ${warn_CFLAGS}])