Bump (Linux) Clang baseline to 5.0.2

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

This no longer sets CLANGVER, CLANG_VERSION, and CLANG_FULL_VERSION when using
Apple Clang (on macOS), which uses different version numbers from upstream
anyway.  But those variables are only used in the context of compiler plugins,
which do not work with Apple Clang anyway (which lacks necessary include files).

(Also, move "AC_SUBST(COM_IS_CLANG)" up to where it belongs.)

Change-Id: Iee37c42ecacf52fa5a07e35241bcd404025e1cdf
Reviewed-on: https://gerrit.libreoffice.org/63899
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

README.md

@@ -40,15 +40,11 @@ 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: GCC 4.8.1 or Clang
+    * Build: either GCC 4.8.1; 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
 
-At least Clang 3.4.2 is known to be too old to pass the configure.ac check "whether $CXX supports
-C++17, C++14, or C++11" in its current form (due to the #pragma GCC diagnostic ignored "-Wpragmas"
-that it does not understand).
-
 If you want to use Clang with the LibreOffice compiler plugins, the minimal
 version of Clang is 5.0.2. Since Xcode doesn't provide the compiler plugin
 headers, you have to compile your own Clang to use them on macOS.

configure.ac

@@ -3639,37 +3639,52 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
     [AC_MSG_RESULT([yes])
      COM_IS_CLANG=TRUE],
     [AC_MSG_RESULT([no])])
+AC_SUBST(COM_IS_CLANG)
 
 CC_PLAIN=$CC
+CLANGVER=
 if test "$COM_IS_CLANG" = TRUE; then
-    AC_MSG_CHECKING([the Clang version])
-    if test "$_os" = WINNT; then
-        dnl In which case, assume clang-cl:
-        my_args="/EP /TC"
-        dnl Filter out -FIIntrin.h, which needs to be explicitly stated for
-        dnl clang-cl:
-        CC_PLAIN=
-        for i in $CC; do
-            case $i in
-            -FIIntrin.h)
-                ;;
-            *)
-                CC_PLAIN="$CC_PLAIN $i"
-                ;;
-            esac
-        done
-    else
-        my_args="-E -P"
+    AC_MSG_CHECKING([whether Clang is new enough])
+    AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
+        #if !defined __apple_build_version__
+        #error
+        #endif
+        ]])],
+        [my_apple_clang=yes],[my_apple_clang=])
+    if test "$my_apple_clang" = yes; then
+        AC_MSG_RESULT([assumed yes (Apple Clang)])
+    else
+        if test "$_os" = WINNT; then
+            dnl In which case, assume clang-cl:
+            my_args="/EP /TC"
+            dnl Filter out -FIIntrin.h, which needs to be explicitly stated for
+            dnl clang-cl:
+            CC_PLAIN=
+            for i in $CC; do
+                case $i in
+                -FIIntrin.h)
+                    ;;
+                *)
+                    CC_PLAIN="$CC_PLAIN $i"
+                    ;;
+                esac
+            done
+        else
+            my_args="-E -P"
+        fi
+        clang_version=`echo __clang_major__.__clang_minor__.__clang_patchlevel__ | $CC_PLAIN $my_args -`
+        CLANG_FULL_VERSION=`echo __clang_version__ | $CC_PLAIN $my_args -`
+        CLANGVER=`echo $clang_version \
+            | $AWK -F. '{ print \$1*10000+(\$2<100?\$2:99)*100+(\$3<100?\$3:99) }'`
+        if test "$CLANGVER" -ge 50002; then
+            AC_MSG_RESULT([yes ($clang_version)])
+        else
+            AC_MSG_ERROR(["$CLANG_FULL_VERSION" is too old or unrecognized, must be at least Clang 5.0.2])
+        fi
+        AC_DEFINE_UNQUOTED(CLANG_VERSION,$CLANGVER)
+        AC_DEFINE_UNQUOTED(CLANG_FULL_VERSION,$CLANG_FULL_VERSION)
     fi
-    clang_version=`echo __clang_major__.__clang_minor__.__clang_patchlevel__ | $CC_PLAIN $my_args -`
-    CLANG_FULL_VERSION=`echo __clang_version__ | $CC_PLAIN $my_args -`
-    CLANGVER=`echo $clang_version \
-        | $AWK -F. '{ print \$1*10000+(\$2<100?\$2:99)*100+(\$3<100?\$3:99) }'`
-    AC_MSG_RESULT([Clang $CLANG_FULL_VERSION, $CLANGVER])
-    AC_DEFINE_UNQUOTED(CLANG_VERSION,$CLANGVER)
-    AC_DEFINE_UNQUOTED(CLANG_FULL_VERSION,$CLANG_FULL_VERSION)
 fi
-AC_SUBST(COM_IS_CLANG)
 
 SHOWINCLUDES_PREFIX=
 if test "$_os" = WINNT; then