meson: conditionally define optional dependencies in pkg-config

lix.pc currently lists several optional libraries (such as libcpuid and
libseccomp) as required dependencies. Update the pkg-config generation
step to only list optional dependencies which are used by the Lix build.

Change-Id: Iceea7ecc9421b5b10fc08edbe95a063a4e39cea1

lix/lix.pc.in

@@ -7,6 +7,6 @@ Description: Lix Package Manager (libraries)
 Version: @PACKAGE_VERSION@
 # dependencies on boost, libarchive, nlohmann_json are omitted since they are optional (only required by some headers)
 Requires: lix-base kj-async @BOEHM_IF_FOUND@
-Requires.private: @AWS_SDK_IF_FOUND@ @BOEHM_IF_FOUND@ libeditline lowdown ncurses libbrotlidec libbrotlienc libcpuid libarchive libcrypto capnp-rpc libcurl libseccomp sqlite3
+Requires.private: @AWS_SDK_IF_FOUND@ @BOEHM_IF_FOUND@ @CPUID_IF_FOUND@ @SECCOMP_IF_FOUND@ libeditline lowdown ncurses libbrotlidec libbrotlienc libarchive libcrypto capnp-rpc libcurl sqlite3
 Cflags: -I${includedir}
 Libs: -L${libdir} -lrust_monocrate -llix

meson.build

@@ -596,6 +596,8 @@ foreach pkgconf : [ 'lix-base.pc', 'lix.pc' ]
       'PACKAGE_VERSION' : meson.project_version(),
       'AWS_SDK_IF_FOUND' : aws_sdk.found() ? 'aws-cpp-sdk-core aws-cpp-sdk-s3 aws-cpp-sdk-transfer' : '',
       'BOEHM_IF_FOUND' : boehm.found() ? 'bdw-gc' : '',
+      'CPUID_IF_FOUND' : cpuid.found() ? 'libcpuid' : '',
+      'SECCOMP_IF_FOUND' : seccomp.found() ? 'libseccomp' : '',
     },
   )
 endforeach