Fix static only build

Make sure to build the shared library only if BUILD_SHARED_LIBS is
ON. Normally, CMake takes care of this automatically, but libcuefile
wants to build both the shared and static variants, so the normal
logic of CMake doesn't apply.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/src/CMakeLists.txt
===================================================================
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -4,7 +4,11 @@
 add_library(cuefile-static STATIC cd cdtext cue_parse cue_print cue_scan cuefile time toc toc_parse toc_print toc_scan)
 set_target_properties(cuefile-static PROPERTIES OUTPUT_NAME cuefile CLEAN_DIRECT_OUTPUT 1)
 
+if (BUILD_SHARED_LIBS)
 add_library(cuefile-shared SHARED cd cdtext cue_parse cue_print cue_scan cuefile time toc toc_parse toc_print toc_scan)
 set_target_properties(cuefile-shared PROPERTIES OUTPUT_NAME cuefile CLEAN_DIRECT_OUTPUT 1 VERSION 0.0.0 SOVERSION 0)
 
 install(TARGETS cuefile-static cuefile-shared LIBRARY DESTINATION "lib${LIB_SUFFIX}" ARCHIVE DESTINATION "lib${LIB_SUFFIX}")
+else (BUILD_SHARED_LIBS)
+install(TARGETS cuefile-static LIBRARY DESTINATION "lib${LIB_SUFFIX}" ARCHIVE DESTINATION "lib${LIB_SUFFIX}")
+endif (BUILD_SHARED_LIBS)