[cmake] Enable static building

OSS Fuzz strongly recommends static linking for tests [1]. Patch enables
static linking for a fuzzing test added in commit
2ad1469612 ("Add fuzzer for
certificate_data_set_pem()").

1. https://google.github.io/oss-fuzz/further-reading/fuzzer-environment/#runtime-dependencies

cmake/ConfigOptions.cmake

@@ -199,6 +199,11 @@ if (BUILD_FUZZERS)
 
     set(BUILD_TESTING ON)
 
+    if (BUILD_SHARED_LIBS STREQUAL "OFF")
+        set(CMAKE_FIND_LIBRARY_SUFFIXES ".a")
+        set(CMAKE_CXX_FLAGS "-static ${CMAKE_CXX_FLAGS}")
+    endif()
+
     # A special target with fuzzer and sanitizer flags.
     add_library(fuzzer_config INTERFACE)