Enable more GCC warnings

This enables the following additional warnings:
-Wold-style-definition
-Warray-bounds=2 instead of the default 1
-Wformat=2, but only for GCC 4.8+ as Wno-format-nonliteral is not
 respected for older versions
-Wunused-const-variable, which enables level 2 instead of the default 1
-Warray-bounds also in debug mode through -ftree-vrp
-Wnull-dereference also in debug mode through
 -fdelete-null-pointer-checks

Closes https://github.com/curl/curl/pull/2747

m4/curl-compilers.m4

@@ -1012,6 +1012,7 @@ AC_DEFUN([CURL_SET_COMPILER_WARNING_OPTS], [
           dnl Only gcc 3.4 or later
           if test "$compiler_num" -ge "304"; then
             tmp_CFLAGS="$tmp_CFLAGS -Wdeclaration-after-statement"
+            tmp_CFLAGS="$tmp_CFLAGS -Wold-style-definition"
           fi
           #
           dnl Only gcc 4.0 or later
@@ -1030,6 +1031,8 @@ AC_DEFUN([CURL_SET_COMPILER_WARNING_OPTS], [
             tmp_CFLAGS="$tmp_CFLAGS -Wmissing-parameter-type -Wempty-body"
             tmp_CFLAGS="$tmp_CFLAGS -Wclobbered -Wignored-qualifiers"
             tmp_CFLAGS="$tmp_CFLAGS -Wconversion -Wno-sign-conversion -Wvla"
+            dnl required for -Warray-bounds, included in -Wall
+            tmp_CFLAGS="$tmp_CFLAGS -ftree-vrp"
           fi
           #
           dnl Only gcc 4.5 or later
@@ -1045,12 +1048,23 @@ AC_DEFUN([CURL_SET_COMPILER_WARNING_OPTS], [
             tmp_CFLAGS="$tmp_CFLAGS -Wdouble-promotion"
           fi
           #
+          dnl only gcc 4.8 or later
+          if test "$compiler_num" -ge "408"; then
+            tmp_CFLAGS="$tmp_CFLAGS -Wformat=2"
+          fi
+          #
+          dnl Only gcc 5 or later
+          if test "$compiler_num" -ge "500"; then
+            tmp_CFLAGS="$tmp_CFLAGS -Warray-bounds=2"
+          fi
+          #
           dnl Only gcc 6 or later
           if test "$compiler_num" -ge "600"; then
             tmp_CFLAGS="$tmp_CFLAGS -Wshift-negative-value"
             tmp_CFLAGS="$tmp_CFLAGS -Wshift-overflow=2"
-            tmp_CFLAGS="$tmp_CFLAGS -Wnull-dereference"
+            tmp_CFLAGS="$tmp_CFLAGS -Wnull-dereference -fdelete-null-pointer-checks"
             tmp_CFLAGS="$tmp_CFLAGS -Wduplicated-cond"
+            tmp_CFLAGS="$tmp_CFLAGS -Wunused-const-variable"
           fi
           #
           dnl Only gcc 7 or later