spec: Rely on default -flto values instead of using auto

According to the GCC docs[1], -flto=auto does:

> Use -flto=auto to use GNU make’s job server, if available, or
> otherwise fall back to autodetection of the number of CPU threads
> present in your system.

clang does not support -flto=auto (or -flto=jobserver for that matter),
so using this option breaks the build with non-gcc compilers. Pass just
-flto instead and rely on gcc doing the right thing.

[1] https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

octave.spec

@@ -22,7 +22,7 @@
 %global relsuf .rc%{?rcver}
 %endif
 
-%global optflags %{optflags} -flto=auto
+%global optflags %{optflags}
 %global build_ldflags %{build_ldflags} -flto
 
 Name:           octave
@@ -240,6 +240,14 @@ export CPPFLAGS=-I%{_includedir}/suitesparse
 # Disable _GLIBCXX_ASSERTIONS for now
 # https://savannah.gnu.org/bugs/?55547
 export CXXFLAGS="$(echo %optflags | sed s/-Wp,-D_GLIBCXX_ASSERTIONS//)"
+
+verstr=$(%{__cxx} --version | head -1)
+if [[ "$verstr" == *"GCC"* ]]; then
+  CXXFLAGS="$CXXFLAGS -flto=auto"
+else
+  CXXFLAGS="$CXXFLAGS -flto"
+fi
+
 %configure --enable-shared --disable-static \
  --enable-float-truncate \
  %{?disabledocs} \