Pass -Wno-unused-but-set-variable by default, not just in make develop

The Bison-generated parser skeleton otherwise triggers this warning
with Apple clang++ for `yynerrs_`.

CMakeLists.txt

@@ -162,10 +162,7 @@ if(MSVC)
   )
   add_definitions(/D_CRT_SECURE_NO_WARNINGS)
 else()
-  add_compile_options(-Wall -pedantic -fno-exceptions -fno-rtti -Wno-unknown-warning-option
-                      # C++20 allows macros to take zero variadic arguments.
-                      # Some versions of Clang don't recognize this, and treat them as a GNU extension.
-                      -Wno-gnu-zero-variadic-macro-arguments)
+  add_compile_options(-Wall -pedantic -Wno-unknown-warning-option -fno-exceptions -fno-rtti)
 
   if(MORE_WARNINGS)
     add_compile_options(-Wextra
@@ -174,11 +171,15 @@ else()
                         -Wstringop-overflow=4 -Wtrampolines -Wundef -Wuninitialized -Wunused -Wshadow
                         -Wformat=2 -Wformat-overflow=2 -Wformat-truncation=1
                         -Wno-format-nonliteral -Wno-strict-overflow
-                        -Wno-unused-but-set-variable # bison's `yynerrs_` is incremented but unused
                         -Wno-type-limits -Wno-tautological-constant-out-of-range-compare
                         -Wvla) # MSVC does not support VLAs
   endif()
 
+  add_compile_options(-Wno-unused-but-set-variable # bison's `yynerrs_` is incremented but unused
+                      # C++20 allows macros to take zero variadic arguments.
+                      # Some versions of Clang don't recognize this, and treat them as a GNU extension.
+                      -Wno-gnu-zero-variadic-macro-arguments)
+
   if(MINGW)
     # MinGW warns about format specifiers like `%z` and `%j`, which are missing on Windows XP and earlier.
     # We rely on runtimes more modern than that, so we can ignore those warnings.

Makefile

@@ -26,7 +26,8 @@ PNGLDLIBS  := `${PKG_CONFIG} --libs-only-l libpng`
 # Note: if this comes up empty, `version.cpp` will automatically fall back to last release number
 VERSION_STRING := `git --git-dir=.git -c safe.directory='*' describe --tags --dirty --always 2>/dev/null`
 
-WARNFLAGS := -Wall -pedantic -Wno-unknown-warning-option -Wno-gnu-zero-variadic-macro-arguments
+WARNFLAGS := -Wall -pedantic -Wno-unknown-warning-option \
+             -Wno-gnu-zero-variadic-macro-arguments -Wno-unused-but-set-variable
 
 # Overridable CXXFLAGS
 CXXFLAGS     ?= -O3 -flto -DNDEBUG