Use libcerf C variant (cerf::cerf) instead of cerfcpp.

Gentoo's sci-libs/libcerf is built with CERF_CPP=OFF, producing libcerf.so
(C variant) but not libcerfcpp.so. Switching BornAgain's two cerfcx call
sites to use the C ABI via a small std::complex<-> _Complex bridge avoids
the need to fork libcerf to also build the C++ variant — std::complex<double>
and _Complex double have identical memory layout on every supported ABI.

verified 2026-05-14 against bornagain-23.0 + libcerf-2.5.

--- a/cmake/BornAgain/FindLibraries.cmake
+++ b/cmake/BornAgain/FindLibraries.cmake
@@ -46,10 +46,10 @@

 list(APPEND BA_Dependencies "${GSL_LIBRARIES}")

-find_package(cerf CONFIG REQUIRED COMPONENTS shared CXX)
-get_property(Cerf_LIBRARIES TARGET cerf::cerfcpp PROPERTY LOCATION)
-message(STATUS "cerf: C++ version = ${cerf_VERSION}, lib=${Cerf_LIBRARIES}")
-set(cerf_target cerf::cerfcpp)
+find_package(cerf CONFIG REQUIRED COMPONENTS shared C)
+get_property(Cerf_LIBRARIES TARGET cerf::cerf PROPERTY LOCATION)
+message(STATUS "cerf: C version = ${cerf_VERSION}, lib=${Cerf_LIBRARIES}")
+set(cerf_target cerf::cerf)

 list(APPEND BA_Dependencies "${Cerf_LIBRARIES}")

--- a/Sim/Computation/RoughMultiLayerContribution.cpp
+++ b/Sim/Computation/RoughMultiLayerContribution.cpp
@@ -20,7 +20,8 @@
 #include "Sample/Interface/Roughness.h"
 #include "Sample/Multilayer/Layer.h"
 #include "Sample/Multilayer/Sample.h"
-#include <cerf.h>
+#include <cerf.h>      // libcerf C variant: cerfcx takes/returns double _Complex
+#include <cstring>
 #include <numbers>

 using std::numbers::pi;
@@ -37,13 +38,25 @@

 namespace {

+// Bridge std::complex<double> <-> double _Complex for the libcerf C ABI.
+// Layout is identical (two adjacent doubles, real then imag) on every
+// supported platform, so a bitwise copy round-trips losslessly.
+inline complex_t cerfcx_cxx(complex_t z)
+{
+    double _Complex cz;
+    std::memcpy(&cz, &z, sizeof(cz));
+    const double _Complex cr = ::cerfcx(cz);
+    complex_t r;
+    std::memcpy(&r, &cr, sizeof(r));
+    return r;
+}
 complex_t h_above(complex_t z)
 {
-    return 0.5 * cerfcx(-mul_I(z) / std::sqrt(2.0));
+    return 0.5 * cerfcx_cxx(-mul_I(z) / std::sqrt(2.0));
 }
 complex_t h_below(complex_t z)
 {
-    return 0.5 * cerfcx(mul_I(z) / std::sqrt(2.0));
+    return 0.5 * cerfcx_cxx(mul_I(z) / std::sqrt(2.0));
 }

 complex_t get_refractive_term(const ReSample& re_sample, size_t i_layer, double wavelength)