Some shader compilers did not seemingly like #if XXX blocks

if XXX was not defined. Changed to #ifdef XXX.

resources/shaders/gouraud.fs

@@ -16,7 +16,7 @@ struct SlopeDetection
 uniform vec4 uniform_color;
 uniform SlopeDetection slope;
 
-#if ENABLE_ENVIRONMENT_MAP
+#ifdef ENABLE_ENVIRONMENT_MAP
     uniform sampler2D environment_tex;
     uniform bool use_environment_tex;
 #endif // ENABLE_ENVIRONMENT_MAP
@@ -44,7 +44,7 @@ void main()
 	vec3 color = slope.actived ? slope_color() : uniform_color.rgb;
     // if the fragment is outside the print volume -> use darker color
 	color = (any(lessThan(delta_box_min, ZERO)) || any(greaterThan(delta_box_max, ZERO))) ? mix(color, ZERO, 0.3333) : color;
-#if ENABLE_ENVIRONMENT_MAP
+#ifdef ENABLE_ENVIRONMENT_MAP
     if (use_environment_tex)
         gl_FragColor = vec4(0.45 * texture2D(environment_tex, normalize(eye_normal).xy * 0.5 + 0.5).xyz + 0.8 * color * intensity.x, uniform_color.a);
     else

src/slic3r/GUI/GLShader.cpp

@@ -61,7 +61,7 @@ bool GLShaderProgram::init_from_files(const std::string& name, const ShaderFilen
     for (std::string_view def : defines)
         // Our shaders are stored with "\r\n", thus replicate the same here for consistency. Likely "\n" would suffice, 
         // but we don't know all the OpenGL shader compilers around.
-        defines_program += format("#define %s 1\r\n", def);
+        defines_program += format("#define %s\r\n", def);
 
     ShaderSources sources = {};
     for (size_t i = 0; i < static_cast<size_t>(EShaderType::Count); ++i) {