Do not undefine __STRICT_ANSI__

The `__STRICT_ANSI__` macro is defined by the compiler and it's undefined to undefine or redefine it.

Using `-U__STRICT_ANSI__ -std=c++11` is just silly. If you don't want strict mode, don't ask for strict mode. Certainly don't ask for strict mode and then undefined the macro that is defined by strict mode.

The correct solution is `-std=gnu++11` which doesn't define the macro in the first place.

xs/Build.PL

@@ -22,14 +22,14 @@ $ENV{LD_RUN_PATH} //= "";
 my @cflags = qw(-D_GLIBCXX_USE_C99 -DHAS_BOOL -DNOGDI -DSLIC3RXS -DBOOST_ASIO_DISABLE_KQUEUE -Dexprtk_disable_rtl_io_file -Dexprtk_disable_return_statement -Dexprtk_disable_rtl_vecops -Dexprtk_disable_string_capabilities -Dexprtk_disable_enhanced_features);
 push @cflags, "-DSLIC3R_BUILD_COMMIT=$ENV{SLIC3R_GIT_VERSION}" if defined $ENV{SLIC3R_GIT_VERSION};
 
+# std=c++11 Enforce usage of C++11 (required now). Minimum compiler supported: gcc 4.9, clang 3.3, MSVC 14.0
 if ($cpp_guess->is_gcc)	{
-	# GCC is pedantic with c++11 std, so undefine strict ansi to get M_PI back
-	push @cflags, qw(-U__STRICT_ANSI__);
+	# GCC is pedantic with c++11 std, so use -std=gnu++11 to be able to use M_PI
+	push @cflags, qw(-std=gnu++11);
+} else {
+	push @cflags, qw(-std=c++11);
 }
 
-# std=c++11 Enforce usage of C++11 (required now). Minimum compiler supported: gcc 4.9, clang 3.3, MSVC 14.0
-push @cflags, qw(-std=c++11);
-
 my @ldflags = ();
 
 if ($linux && (defined $ENV{SLIC3R_STATIC} && $ENV{SLIC3R_STATIC})) {