Bump (Linux) GCC baseline to 7.0.0

...as discussed at
<https://lists.freedesktop.org/archives/libreoffice/2018-November/081435.html>
"minutes of ESC call ..."

Change-Id: Ib1a4fdc56c51ab5c9e45173263689db2b88d72e7
Reviewed-on: https://gerrit.libreoffice.org/63951
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

README.md

@@ -40,7 +40,7 @@ run and compile LibreOffice, also used by the TDF builds:
     * Build: 10.13.2 + Xcode 9.3
 * Linux:
     * Runtime: RHEL 6 or CentOS 6
-    * Build: either GCC 4.8.1; or Clang 5.0.2 with libstdc++ 7.3.0
+    * Build: either GCC 7.0.0; or Clang 5.0.2 with libstdc++ 7.3.0
 * iOS (only for LibreOfficeKit):
     * Runtime: 11.4 (only support for newer i devices == 64 bit)
     * Build: Xcode 9.3 and iPhone SDK 11.4

configure.ac

@@ -3796,8 +3796,8 @@ if test "$GCC" = "yes" -a -z "$COM_IS_CLANG"; then
 
     AC_MSG_RESULT([gcc $_gcc_version ($gcc_full_version)])
 
-    if test "$gcc_full_version" -lt 40801; then
-        AC_MSG_ERROR([GCC $_gcc_version is too old, must be at least GCC 4.8.1])
+    if test "$gcc_full_version" -lt 70000; then
+        AC_MSG_ERROR([GCC $_gcc_version is too old, must be at least GCC 7.0.0])
     fi
 else
     # Explicitly force GCC_VERSION to be empty, even for Clang, to check incorrect uses.