diff --git a/setup/extensions.py b/setup/extensions.py
index c2b8467300..e3283b0ea3 100644
--- a/setup/extensions.py
+++ b/setup/extensions.py
@@ -134,8 +134,11 @@ if isosx:
 if isunix:
     cc = os.environ.get('CC', 'gcc')
     cxx = os.environ.get('CXX', 'g++')
-    cflags = '-O3 -Wall -DNDEBUG -fPIC -fno-strict-aliasing -pipe'.split()
-    ldflags = ['-Wall']
+    cflags = os.environ.get('OVERRIDE_CFLAGS',
+        '-O3 -Wall -DNDEBUG -fno-strict-aliasing -pipe')
+    cflags = shlex.split(cflags) + '-fPIC'
+    ldflags = os.environ.get('OVERRIDE_LDFLAGS', '-Wall')
+    ldflags = shlex.split(ldflags)
     cflags += shlex.split(os.environ.get('CFLAGS', ''))
     ldflags += shlex.split(os.environ.get('LDFLAGS', ''))
 
diff --git a/setup/install.py b/setup/install.py
index fb17049e4c..1dbbb3f8a5 100644
--- a/setup/install.py
+++ b/setup/install.py
@@ -200,6 +200,12 @@ class Install(Develop):
             <prefix>/share/calibre. These can all be controlled via options.
 
             The default <prefix> is the prefix of your python installation.
+
+            The .desktop, .mime and icon files are installed using XDG. The
+            location they are installed to can be controlled by setting
+            the environment variables:
+            XDG_DATA_DIRS=/usr/share equivalent
+            XDG_UTILS_INSTALL_MODE=system
     ''')
     short_description = 'Install calibre from source'