bpo-30104: configure now detects when cc is clang (#1233)

Detect when the "cc" compiler (and the $CC variable) is the Clang
compiler. The test is needed to add the -fno-strict-aliasing option
on FreeBSD where cc is clang.

configure

@@ -6812,18 +6812,31 @@ then
            WRAP="-fwrapv"
         fi
 
-        # Clang also needs -fwrapv
         case $CC in
             *clang*)
+                cc_is_clang=1
+                ;;
+            *)
+                if $CC --version 2>&1 | grep -q clang
+                then
+                    cc_is_clang=1
+                else
+                    cc_is_clang=
+                fi
+        esac
+
+        if test -n "${cc_is_clang}"
+        then
+            # Clang also needs -fwrapv
             WRAP="-fwrapv"
+
             # bpo-30104: Python/dtoa.c requires to be build with
             # -fno-strict-aliasing to fix compiler issue on the
             # double/ULong[2] union using clang 4.0 and optimization level
             # -O2 or higher
             # https://bugs.llvm.org//show_bug.cgi?id=31928
             ALIASING="-fno-strict-aliasing"
-                ;;
-        esac
+        fi
 
 	case $ac_cv_prog_cc_g in
 	yes)

configure.ac

@@ -1452,18 +1452,31 @@ then
            WRAP="-fwrapv"
         fi
 
-        # Clang also needs -fwrapv
         case $CC in
             *clang*)
+                cc_is_clang=1
+                ;;
+            *)
+                if $CC --version 2>&1 | grep -q clang
+                then
+                    cc_is_clang=1
+                else
+                    cc_is_clang=
+                fi
+        esac
+
+        if test -n "${cc_is_clang}"
+        then
+            # Clang also needs -fwrapv
             WRAP="-fwrapv"
+
             # bpo-30104: Python/dtoa.c requires to be build with
             # -fno-strict-aliasing to fix compiler issue on the
             # double/ULong[2] union using clang 4.0 and optimization level
             # -O2 or higher
             # https://bugs.llvm.org//show_bug.cgi?id=31928
             ALIASING="-fno-strict-aliasing"
-                ;;
-        esac
+        fi
 
 	case $ac_cv_prog_cc_g in
 	yes)