src/CMakeLists.txt: fix static build

Add BUILD_SHARED_LIBS and BUILD_STATIC_LIBS to allow the user to build a
static only version otherwise build will fail on:

[ 93%] Building C object example/CMakeFiles/example.dir/example.c.o
[100%] Linking C executable example
/home/giuliobenetti/autobuild/run/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/i686-buildroot-linux-uclibc/8.3.0/../../../../i686-buildroot-linux-uclibc/bin/ld: attempted static link of dynamic object `../src/libuwsc.so.3.3.2'
collect2: error: ld returned 1 exit status
example/CMakeFiles/example.dir/build.make:87: recipe for target 'example/example' failed

Fixes:
 - http://autobuild.buildroot.org/results/eb7b73568ee4058f381c3e7c5634b1c5d92c3ca4

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

example/CMakeLists.txt

@@ -1,4 +1,8 @@
 include_directories(${CMAKE_SOURCE_DIR}/src ${CMAKE_SOURCE_DIR}/src/buffer ${CMAKE_BINARY_DIR}/src ${LIBEV_INCLUDE_DIR})
 
 add_executable(example example.c)
-target_link_libraries(example uwsc ${LIBEV_LIBRARY})
+if(BUILD_SHARED_LIBS)
+    target_link_libraries(example uwsc ${LIBEV_LIBRARY})
+else()
+    target_link_libraries(example uwsc_s ${LIBEV_LIBRARY})
+endif()

src/CMakeLists.txt

@@ -14,6 +14,13 @@ include_directories(${CMAKE_CURRENT_SOURCE_DIR} ${CMAKE_CURRENT_SOURCE_DIR}/buff
 set(EXTRA_LIBS ${LIBEV_LIBRARY} dl)
 set(SOURCE_FILES uwsc.c log.c utils.c buffer/buffer.c sha1.c ssl.c)
 
+option(BUILD_SHARED_LIBS "Build shared library" ON)
+option(BUILD_STATIC_LIBS "Build static library" ON)
+
+if(NOT BUILD_SHARED_LIBS AND NOT BUILD_STATIC_LIBS)
+    message(FATAL_ERROR "BUILD_SHARED_LIBS and BUILD_STATIC_LIBS can't be OFF at the same time, please select at least one of them")
+endif()
+
 set(UWSC_SSL_SUPPORT_CONFIG 1)
 option(UWSC_SSL_SUPPORT "SSL support" ON)
 
@@ -89,13 +96,25 @@ else()
     endif()
 endif()
 
-add_library(uwsc SHARED ${SOURCE_FILES})
-set_target_properties(uwsc PROPERTIES VERSION ${UWSC_VERSION_MAJOR}.${UWSC_VERSION_MINOR}.${UWSC_VERSION_PATCH})
-target_link_libraries(uwsc ${EXTRA_LIBS})
+if(BUILD_SHARED_LIBS)
+    add_library(uwsc SHARED ${SOURCE_FILES})
+    set_target_properties(uwsc PROPERTIES VERSION ${UWSC_VERSION_MAJOR}.${UWSC_VERSION_MINOR}.${UWSC_VERSION_PATCH})
+    target_link_libraries(uwsc ${EXTRA_LIBS})
+    install(
+        TARGETS uwsc
+        LIBRARY DESTINATION lib
+    )
+endif()
 
-add_library(uwsc_s STATIC ${SOURCE_FILES})
-set_target_properties(uwsc_s PROPERTIES OUTPUT_NAME uwsc)
-target_link_libraries(uwsc_s ${EXTRA_LIBS})
+if(BUILD_STATIC_LIBS)
+    add_library(uwsc_s STATIC ${SOURCE_FILES})
+    set_target_properties(uwsc_s PROPERTIES OUTPUT_NAME uwsc)
+    target_link_libraries(uwsc_s ${EXTRA_LIBS})
+    install(
+        TARGETS uwsc_s
+        ARCHIVE DESTINATION lib
+    )
+endif()
 
 # configure a header file to pass some of the CMake settings to the source code
 configure_file(${CMAKE_CURRENT_SOURCE_DIR}/config.h.in ${CMAKE_CURRENT_BINARY_DIR}/config.h)
@@ -114,11 +133,5 @@ install(
         include/uwsc
 )
 
-install(
-    TARGETS uwsc uwsc_s
-    LIBRARY DESTINATION lib
-    ARCHIVE DESTINATION lib
-)
-
 message(STATUS "UWSC_VERSION: ${UWSC_VERSION_MAJOR}.${UWSC_VERSION_MINOR}.${UWSC_VERSION_PATCH}")
 message(STATUS "UWSC_SSL_SUPPORT: ${SSL_NAME}")