GLMHelpers: Fix build with C++20 on GCC.

When enabling C++20 the lerp function seems to be redefined on GCC (not
on MSVC), don't redefine it using a CMake definition.

libraries/shared/src/GLMHelpers.h

@@ -212,9 +212,11 @@ T toNormalizedDeviceScale(const T& value, const T& size) {
 #define ROLL(euler) euler.z
 
 // float - linear interpolate
+#if !defined(DONT_REDEFINE_LERP)
 inline float lerp(float x, float y, float a) {
     return x * (1.0f - a) + (y * a);
 }
+#endif
 
 // vec2 lerp - linear interpolate
 template<typename T, glm::precision P>

plugins/openxr/CMakeLists.txt

@@ -22,6 +22,9 @@ include_hifi_library_headers(script-engine)
 if(CMAKE_CXX_COMPILER_ID MATCHES "GNU")
     # Silence GCC warnings
     target_compile_options(openxr PRIVATE -Wno-missing-field-initializers)
+
+    # Fix build issue where lerp is already defined on C++20 / GCC
+    target_compile_definitions(openxr PRIVATE -DDONT_REDEFINE_LERP)
 endif()
 
 set_property(TARGET openxr PROPERTY CXX_STANDARD 20)