Fix PATH for other PLATFORM_OS's (#1671)

The Raylib root Makefile for PLATFORM=PLATFORM_WEB is assumed to
run in Windows environments, which makes it incorrectly set the
PATH variable, so Linux (and possibly other) environments are not
able to find binaries from EMSDK_PATH.

Fix this by checking which PLATFORM_OS and configure PATH
accordingly.

src/Makefile

@@ -162,7 +162,12 @@ ifeq ($(PLATFORM),PLATFORM_WEB)
     CLANG_PATH          = $(EMSDK_PATH)/upstream/bin
     PYTHON_PATH         = $(EMSDK_PATH)/python/3.9.2-1_64bit
     NODE_PATH           = $(EMSDK_PATH)/node/14.15.5_64bit/bin
-    export PATH         = $(EMSDK_PATH);$(EMSCRIPTEN_PATH);$(CLANG_PATH);$(NODE_PATH);$(PYTHON_PATH);C:\raylib\MinGW\bin:$$(PATH)
+
+    ifeq ($(PLATFORM_OS),WINDOWS)
+        export PATH = $(EMSDK_PATH);$(EMSCRIPTEN_PATH);$(CLANG_PATH);$(NODE_PATH);$(PYTHON_PATH);C:\raylib\MinGW\bin:$$(PATH)
+    else
+        export PATH := $(EMSDK_PATH):$(EMSCRIPTEN_PATH):$(CLANG_PATH):$(NODE_PATH):$(PYTHON_PATH):$(PATH)
+    endif
 endif
 
 ifeq ($(PLATFORM),PLATFORM_ANDROID)