demo/sdl3_renderer: Rewrite SDL3 Makefile to support Emscripten (#894)

* fix(demo/sdl3_renderer): Makefile should now support Win + Emsdk

Fixed several issues that would prevent this demo from being compilable
on Windows with Emscripten: (1) there was a typo preventing proper
resolution of BINEXT, (2) in case of using Emscripten, Windows was taking
a precedense due to OS always being defined, (3) BINEXT now has a
special case for Emscipten, (4) pkg-config provided by w64devkit has no
--path flag, so we retry with --exists too, (5) compiler invocation was
missing system flags, which could break on some MSYS2 runtimes,
(6) compiler invocation now uses all kinds of C/CPP/LD flags so you can
easily inject something like --shell-file now, and probably more...

This was done in order to help with implementing and testing for:
#888

* refactor(demo/sdl3_renderer): consistent syntax and flow in Makefile

Use ${} for variable expansion and $() for invoking functions.
Use $(or ...) for all branches instead of ifeq(...) blocks.
Hopefully, this will make it easier to read and understand.

* fix(demo/sdl3_renderer): append user provided C/CPP/LD flags in Makefile

Before it was overwriting those flags entirely, meaning that it could
delete some required ones (e.g. from pkg-config). Now it will append
them correctly, so your flags will take priority where it matters.

Author: sleeptightAnsiC

demo/sdl3_renderer/Makefile

@@ -1,41 +1,56 @@
 # this Makefile is specific to GNU Make and GCC'compatible compilers
 
-PKG_CONFIG ?= $(shell command -v pkg-config)
-ifeq (,$(PKG_CONFIG))
-    $(error missing pkg-config utility!)
-endif
+PKG_CONFIG := $(or\
+    $(shell pkg-config --version >/dev/null 2>/dev/null && echo pkg-config),\
+    $(shell command -v pkg-config 2>/dev/null),\
+    $(error missing pkg-config utility!))
 
-PKG_SDL3 ?= sdl3
-ifeq (,$(shell $(PKG_CONFIG) --path $(PKG_SDL3)))
-    $(error $(PKG_CONFIG) could not find: $(PKG_SDL3))
-endif
+PKG_SDL3 := $(or\
+    $(and $(shell ${PKG_CONFIG} sdl3 --path 2>/dev/null),sdl3),\
+    $(shell ${PKG_CONFIG} sdl3 --exists 2>/dev/null && echo sdl3),\
+    $(error pkg-config was unable to find: sdl3))
 
-OS ?= $(shell uname -s)
-BINEXT-Windows_NT = .exe
-BINEXT ?= $(BINEXIT-$(OS))
+OSNAME := $(or\
+    $(and ${EMSCRIPTEN}, Emscripten),\
+    ${OS},\
+    $(shell uname -s),\
+    $(error could not detect OSNAME))
 
-TEMPDIR ?= ./bin
-BIN ?= $(TEMPDIR)/demo$(BINEXT)
+binext_Windows_NT := .exe
+binext_Emscripten := .html
+BINEXT := ${binext_${OSNAME}}
 
-CFLAGS += -std=c89 -Wall -Wextra -Wpedantic
-CFLAGS += -O2
-#CFLAGS += -O0 -g
-#CFLAGS += -fsanitize=address
-#CFLAGS += -fsanitize=undefined
-CFLAGS += $(shell $(PKG_CONFIG) $(PKG_SDL3) --cflags)
+TEMPDIR := ./bin
+BIN := ${TEMPDIR}/demo${BINEXT}
 
-LIBS += -lm
-LIBS += $(shell $(PKG_CONFIG) $(PKG_SDL3) --libs)
+cflags += -std=c89 -Wall -Wextra -Wpedantic
+cflags += -O2
+#cflags += -O0 -g
+#cflags += -fsanitize=address
+#cflags += -fsanitize=undefined
+cflags += ${CFLAGS}
 
-DEP ?= $(BIN).d
+cppflags += $(shell ${PKG_CONFIG} ${PKG_SDL3} --cflags --keep-system-cflags)
+cppflags += ${CPPFLAGS}
 
-SRC = main.c
+ldflags += $(shell ${PKG_CONFIG} ${PKG_SDL3} --libs-only-L --libs-only-other --keep-system-libs)
+ldflags += ${LDFLAGS}
 
-$(BIN):
+ldlibs += $(shell ${PKG_CONFIG} ${PKG_SDL3} --libs-only-l --keep-system-libs)
+ldlibs += -lm
+ldlibs += ${LDLIBS}
+# HACK: this one is for compatibility with other demos
+ldlibs += ${LIBS}
+
+DEP := ${TEMPDIR}/$(notdir ${BIN}).d
+
+SRC := main.c
+
+${BIN}:
 	mkdir -p $(dir $@)
-	$(CC) $(SRC) -o $@ -MD -MF $(DEP) $(CFLAGS) $(LIBS)
+	${CC} ${SRC} -o $@ -MD -MF ${DEP} ${cppflags} ${ldflags} ${ldlibs} ${cflags}
 
-$(BIN): $(SRC) ./Makefile ./nuklear_sdl3_renderer.h ./../../nuklear.h
+${BIN}: ${SRC}
 
--include $(DEP)
+-include ${DEP}