fix premake build for linux, thanks to @consultit

See also issue #1538

src/Bullet3Collision/premake4.lua

@@ -6,6 +6,9 @@
 		
 	includedirs {".."}
 
+    if os.is("Linux") then
+        buildoptions{"-fPIC"}
+    end
 
 	files {
 		"**.cpp",

src/Bullet3Common/premake4.lua

@@ -4,6 +4,10 @@
 				
 	kind "StaticLib"
 		
+	if os.is("Linux") then
+	    buildoptions{"-fPIC"}
+	end
+
 	includedirs {".."}
 
 	files {

src/Bullet3Dynamics/premake4.lua

@@ -8,6 +8,9 @@
 		".."
 	}		
 	
+    if os.is("Linux") then
+        buildoptions{"-fPIC"}
+    end
 
 	files {
 		"**.cpp",

src/Bullet3Geometry/premake4.lua

@@ -6,6 +6,9 @@
 		
 	includedirs {".."}
 	
+    if os.is("Linux") then
+        buildoptions{"-fPIC"}
+    end
 
 	files {
 		"**.cpp",

src/Bullet3OpenCL/premake4.lua

@@ -9,6 +9,9 @@ function createProject(vendor)
 			
 		kind "StaticLib"
 		
+        if os.is("Linux") then
+            buildoptions{"-fPIC"}
+        end
 		
 		includedirs {
 			".",".."

src/Bullet3Serialize/Bullet2FileLoader/premake4.lua

@@ -5,6 +5,10 @@
 	includedirs {
 		"../../../src"
 	}
+	
+    if os.is("Linux") then
+        buildoptions{"-fPIC"}
+    end
 	 
 	files {
 		"**.cpp",

src/BulletCollision/premake4.lua

@@ -1,6 +1,9 @@
 	project "BulletCollision"
 
 	kind "StaticLib"
+    if os.is("Linux") then
+        buildoptions{"-fPIC"}
+    end
 	includedirs {
 		"..",
 	}

src/BulletDynamics/premake4.lua

@@ -3,6 +3,9 @@
 	includedirs {
 		"..",
 	}
+    if os.is("Linux") then
+        buildoptions{"-fPIC"}
+    end
 	files {
 		"Dynamics/*.cpp",
                 "Dynamics/*.h",

src/BulletInverseDynamics/premake4.lua

@@ -1,6 +1,9 @@
 	project "BulletInverseDynamics"
 
 	kind "StaticLib"
+    if os.is("Linux") then
+        buildoptions{"-fPIC"}
+    end
 	includedirs {
 		"..",
 	}

src/BulletSoftBody/premake4.lua

@@ -5,6 +5,9 @@
 	includedirs {
 		"..",
 	}
+    if os.is("Linux") then
+        buildoptions{"-fPIC"}
+    end
 	files {
 		"**.cpp",
 		"**.h"

src/LinearMath/premake4.lua

@@ -1,6 +1,9 @@
 	project "LinearMath"
 
 	kind "StaticLib"
+	if os.is("Linux") then
+	    buildoptions{"-fPIC"}
+	end
 	includedirs {
 		"..",
 	}