Fix compilation with GCC 9

Makefile

@@ -31,7 +31,8 @@ WARNFLAGS := -Wall -pedantic -Wno-unknown-warning-option -Wno-gnu-zero-variadic-
 # Overridable CXXFLAGS
 CXXFLAGS     ?= -O3 -flto -DNDEBUG
 # Non-overridable CXXFLAGS
-REALCXXFLAGS := ${CXXFLAGS} ${WARNFLAGS} -std=c++20 -I include -fno-exceptions -fno-rtti
+# GCC 9 doesn't support `-std=c++20`, and all later GCCs treat it as equivalent to `-std=c++2a`.
+REALCXXFLAGS := ${CXXFLAGS} ${WARNFLAGS} -std=c++2a -I include -fno-exceptions -fno-rtti
 # Overridable LDFLAGS
 LDFLAGS      ?=
 # Non-overridable LDFLAGS

include/util.hpp

@@ -50,14 +50,14 @@ char const *printChar(int c);
 
 struct Uppercase {
 	// FNV-1a hash of an uppercased string
-	constexpr size_t operator()(std::string const &str) const {
+	size_t operator()(std::string const &str) const {
 		return std::accumulate(RANGE(str), size_t(0x811C9DC5), [](size_t hash, char c) {
 			return (hash ^ toUpper(c)) * 16777619;
 		});
 	}
 
 	// Compare two strings without case-sensitivity (by converting to uppercase)
-	constexpr bool operator()(std::string const &str1, std::string const &str2) const {
+	bool operator()(std::string const &str1, std::string const &str2) const {
 		return std::equal(RANGE(str1), RANGE(str2), [](char c1, char c2) {
 			return toUpper(c1) == toUpper(c2);
 		});

src/asm/fixpoint.cpp

@@ -5,10 +5,10 @@
 #include "asm/fixpoint.hpp"
 
 #include <math.h>
-#include <numbers>
 #include <stdint.h>
 
-static constexpr double tau = std::numbers::pi * 2;
+// Ideally we'd use `std::numbers::pi`, but GCC 9 doesn't support it.
+static constexpr double tau = 3.141592653589793238462643383279502884L * 2;
 
 static double fix2double(int32_t i, int32_t q) {
 	return i / pow(2.0, q);