external: import local copy of GLM 0.9.7.2

This allows building against the same version of GLM across all targets
and not deal with API changes of newer GLM versions which introduce
regressions. Long term we want to migrate away from GLM and stick to a
minimal self-written subset.

external/glm/.gitignore

@@ -0,0 +1,47 @@
+# Compiled Object files
+*.slo
+*.lo
+*.o
+*.obj
+
+# Precompiled Headers
+*.gch
+*.pch
+
+# Compiled Dynamic libraries
+*.so
+*.dylib
+*.dll
+
+# Fortran module files
+*.mod
+
+# Compiled Static libraries
+*.lai
+*.la
+*.a
+*.lib
+
+# Executables
+*.exe
+*.out
+*.app
+
+# CMake
+CMakeCache.txt
+CMakeFiles
+Makefile
+cmake_install.cmake
+install_manifest.txt
+*.cmake
+# ^ May need to add future .cmake files as exceptions
+
+# Test logs
+Testing/*
+
+# Test input
+test/gtc/*.dds
+
+# Project Files
+*.cbp
+*.user