external/glm: Avoid -std=c++2a -Werror,-Wdeprecated-volatile

...as seen with recent Clang 10 trunk clang-cl:

> In file included from vcl/source/gdi/salgdilayout.cxx:24:
> In file included from vcl/inc\openglgdiimpl.hxx:31:
> In file included from vcl/inc/opengl/RenderList.hxx:16:
> In file included from workdir/UnpackedTarball/glm\glm/glm.hpp:98:
> In file included from workdir/UnpackedTarball/glm\glm/./core/type.hpp:32:
> In file included from workdir/UnpackedTarball/glm\glm/core/type_half.hpp:116:
> workdir/UnpackedTarball/glm\glm/core/type_half.inl(43,6): error: compound assignment to object of volatile-qualified type 'volatile float' is deprecated [-Werror,-Wdeprecated-volatile]
>                         f *= f;             // this will overflow before
>                           ^

Change-Id: I97fdfc0650344db1401cdce126f6755df00c5a0e
Reviewed-on: https://gerrit.libreoffice.org/81427
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

2 files changed, 12 insertions, 0 deletions

diff --git a/external/glm/UnpackedTarball_glm.mk b/external/glm/UnpackedTarball_glm.mk
index 71f1c3cd8ff8..940d12372b94 100644
--- a/external/glm/UnpackedTarball_glm.mk
+++ b/external/glm/UnpackedTarball_glm.mk
@@ -20,6 +20,7 @@ $(eval $(call gb_UnpackedTarball_add_patches,glm, \
     external/glm/Wsign-compare.patch.0 \
     external/glm/Wunused-parameter.patch \
     external/glm/clang-cl.patch.0 \
+    external/glm/c++20.patch.0 \
 ))
 
 # vim: set noet sw=4 ts=4:
diff --git a/external/glm/c++20.patch.0 b/external/glm/c++20.patch.0
new file mode 100755
index 000000000000..559adde48946
--- /dev/null
+++ b/external/glm/c++20.patch.0
@@ -0,0 +1,11 @@
+--- glm/core/type_half.inl
++++ glm/core/type_half.inl
+@@ -40,7 +40,7 @@
+ 		volatile float f = 1e10;

+ 

+ 		for(int i = 0; i < 10; ++i)	

+-			f *= f;             // this will overflow before

++			f = f * f;          // this will overflow before

+ 								// the for­loop terminates

+ 		return f;

+ 	}