Respect user's CFLAGS

--- a/Makefile.am
+++ b/Makefile.am
@@ -2,7 +2,7 @@
 if DEBUG
 export OPTIMISATION = -DDEBUG -ggdb -fstack-protector-explicit
 else
-export OPTIMISATION = -DNDEBUG -Ofast -g
+export OPTIMISATION = -DNDEBUG
 endif
 
 export WARNINGS = -Wall -Wextra -Wconversion -Wsign-compare -Wsign-conversion -Wpedantic
--- a/configure.ac
+++ b/configure.ac
@@ -19,8 +19,6 @@ AC_CANONICAL_HOST
 AC_DEFINE_UNQUOTED([HOST_OS], ["$host_os"], [Host operating system])
 
 # compiler flags
-CFLAGS=""
-CXXFLAGS=""
 
 # Checks for programs
 AC_PROG_CC