Increase template depth

Clang's default value for maximum template depth is 256, which is not enough for ScalFMM.
Set a default compiler option to GCC's default value.

CMakeLists.txt

@@ -165,6 +165,11 @@ if (MORSE_DISTRIB_DIR OR EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/CMakeModules/morse_
   #  -xHost -mfpmath=sse
   # -Wall Wnosign-conversion
   #
+
+  # Set a fixed template depth
+  # Compilers don't use the same default for template-depth, we can enforce the same one everywhere.
+  # The magic number comes from GCC's default: https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Dialect-Options.html#C_002b_002b-Dialect-Options
+  set(SCALFMM_CXX_FLAGS "${SCALFMM_CXX_FLAGS} -ftemplate-depth=900")
   if(CMAKE_CXX_COMPILER_ID STREQUAL "Intel")
     # INTEL
     IF (APPLE)