configure: compile-time SIZEOF checks

... instead of exeucting code to get the size. Removes the use of
LD_LIBRARY_PATH for this.

Fixes #2586
Closes #2589
Reported-by: Bernhard Walle

configure.ac

@@ -2348,7 +2348,7 @@ if test -z "$ssl_backends" -o "x$OPT_CYASSL" != xno; then
       check_for_ca_bundle=1
 
       dnl cyassl/ctaocrypt/types.h needs SIZEOF_LONG_LONG defined!
-      AC_CHECK_SIZEOF(long long)
+      AX_COMPILE_CHECK_SIZEOF(long long)
 
       dnl Versions since at least 2.6.0 may have options.h
       AC_CHECK_HEADERS(cyassl/options.h)
@@ -3449,16 +3449,16 @@ AC_HEADER_TIME
 CURL_CHECK_STRUCT_TIMEVAL
 CURL_VERIFY_RUNTIMELIBS
 
-AC_CHECK_SIZEOF(size_t)
-AC_CHECK_SIZEOF(long)
-AC_CHECK_SIZEOF(int)
-AC_CHECK_SIZEOF(short)
-AC_CHECK_SIZEOF(time_t)
-AC_CHECK_SIZEOF(off_t)
+AX_COMPILE_CHECK_SIZEOF(size_t)
+AX_COMPILE_CHECK_SIZEOF(long)
+AX_COMPILE_CHECK_SIZEOF(int)
+AX_COMPILE_CHECK_SIZEOF(short)
+AX_COMPILE_CHECK_SIZEOF(time_t)
+AX_COMPILE_CHECK_SIZEOF(off_t)
 
 o=$CPPFLAGS
 CPPFLAGS="-I$srcdir/include $CPPFLAGS"
-AC_CHECK_SIZEOF(curl_off_t, unused , [
+AX_COMPILE_CHECK_SIZEOF(curl_off_t, [
 #include <curl/system.h>
 ])
 CPPFLAGS=$o