Some minor fixes for packaging

See https://github.com/gnuradio/volk/pull/100

src/algorithms/libs/volk_gnsssdr_module/volk_gnsssdr/include/volk_gnsssdr/constants.h

@@ -28,11 +28,11 @@
 
 __VOLK_DECL_BEGIN
 
-VOLK_API char* volk_gnsssdr_prefix();
-VOLK_API char* volk_gnsssdr_version();
-VOLK_API char* volk_gnsssdr_c_compiler();
-VOLK_API char* volk_gnsssdr_compiler_flags();
-VOLK_API char* volk_gnsssdr_available_machines();
+VOLK_API const char* volk_gnsssdr_prefix();
+VOLK_API const char* volk_gnsssdr_version();
+VOLK_API const char* volk_gnsssdr_c_compiler();
+VOLK_API const char* volk_gnsssdr_compiler_flags();
+VOLK_API const char* volk_gnsssdr_available_machines();
 
 __VOLK_DECL_END
 

src/algorithms/libs/volk_gnsssdr_module/volk_gnsssdr/lib/CMakeLists.txt

@@ -499,7 +499,7 @@ endif()
 message(STATUS "Loading version ${VERSION} into constants...")
 
 #double escape for windows backslash path separators
-string(REPLACE "\\" "\\\\" prefix ${prefix})
+string(REPLACE "\\" "\\\\" prefix "${prefix}")
 
 configure_file(
     ${CMAKE_CURRENT_SOURCE_DIR}/constants.c.in

src/algorithms/libs/volk_gnsssdr_module/volk_gnsssdr/lib/constants.c.in

@@ -21,33 +21,36 @@
 #include <config.h>
 #endif
 
+#include <stdlib.h>
 #include <volk_gnsssdr/constants.h>
 
-char*
+const char*
 volk_gnsssdr_prefix()
 {
+  const char *prefix = getenv("VOLK_GNSSSDR_PREFIX");
+  if (prefix != NULL) return prefix;
   return "@prefix@";
 }
 
-char*
+const char*
 volk_gnsssdr_version()
 {
   return "@VERSION@";
 }
 
-char*
+const char*
 volk_gnsssdr_c_compiler()
 {
   return "@cmake_c_compiler_version@";
 }
 
-char*
+const char*
 volk_gnsssdr_compiler_flags()
 {
   return "@COMPILER_INFO@";
 }
 
-char*
+const char*
 volk_gnsssdr_available_machines()
 {
   return "@available_machines@";