option to not download libyaml

CMakeLists.txt

@@ -7,15 +7,28 @@ FortranCInterface_VERIFY()
 
 set(CMAKE_Fortran_MODULE_DIRECTORY "${CMAKE_BINARY_DIR}/modules")
 
-# dependencies
-include(cmake/CPM.cmake)
-CPMAddPackage(
-  NAME libyaml 
-  VERSION 0.2.5
-  GITHUB_REPOSITORY "yaml/libyaml"
-  GIT_TAG "release/0.2.5"
-  EXCLUDE_FROM_ALL ON
-)
+option(DOWNLOAD_LIBYAML "If ON, then libyaml will be downloaded and 
+built." ON)
+
+# Find libyaml
+if (DOWNLOAD_LIBYAML) 
+  # use CPM to download and build libyaml
+  include(cmake/CPM.cmake)
+  CPMAddPackage(
+    NAME libyaml 
+    VERSION 0.2.5
+    GITHUB_REPOSITORY "yaml/libyaml"
+    GIT_TAG "release/0.2.5"
+    EXCLUDE_FROM_ALL ON
+  )
+else()
+  # Try to find libyaml using pkg-config
+  find_package(PkgConfig REQUIRED)
+  pkg_check_modules(YAML REQUIRED IMPORTED_TARGET yaml-0.1)
+  if (NOT (${YAML_VERSION} STREQUAL "0.2.5"))
+    message(FATAL_ERROR "PkgConfig found yaml version ${YAML_VERSION} but version 0.2.5 is required")
+  endif()
+endif()
 
 add_subdirectory(src)
 add_subdirectory(tests)

src/CMakeLists.txt

@@ -1,7 +1,10 @@
-set(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/lib")
 
 add_library(libyaml_interface libyaml_interface.c)
-target_link_libraries(libyaml_interface yaml)
+if (DOWNLOAD_LIBYAML)
+  target_link_libraries(libyaml_interface yaml)
+else()
+  target_link_libraries(libyaml_interface PkgConfig::YAML)
+endif()
 
 add_library(fortran-yaml-c 
   fortran_yaml_c_types.f90 
@@ -17,6 +20,3 @@ if ("${CMAKE_Fortran_COMPILER_ID}" MATCHES "GNU")
   endif()
 endif()
 
-install(TARGETS yaml libyaml_interface fortran-yaml-c DESTINATION lib)
-install(DIRECTORY ${CMAKE_BINARY_DIR}/modules DESTINATION .)
-