Add include guard to fftw3 in library and compile tests (#768)

include/boost/math/interpolators/detail/cardinal_trigonometric_detail.hpp

@@ -8,13 +8,20 @@
 #include <cstddef>
 #include <cmath>
 #include <stdexcept>
-#include <fftw3.h>
 #include <boost/math/constants/constants.hpp>
 
 #ifdef BOOST_HAS_FLOAT128
 #include <quadmath.h>
 #endif
 
+#ifdef __has_include
+#  if __has_include(<fftw3.h>)
+#    include <fftw3.h>
+#  else
+#    error "This feature is unavailable without fftw3 installed"
+#endif
+#endif
+
 namespace boost { namespace math { namespace interpolators { namespace detail {
 
 template<typename Real>

include/boost/math/special_functions/chebyshev_transform.hpp

@@ -7,7 +7,6 @@
 #define BOOST_MATH_SPECIAL_CHEBYSHEV_TRANSFORM_HPP
 #include <cmath>
 #include <type_traits>
-#include <fftw3.h>
 #include <boost/math/constants/constants.hpp>
 #include <boost/math/special_functions/chebyshev.hpp>
 
@@ -15,6 +14,14 @@
 #include <quadmath.h>
 #endif
 
+#ifdef __has_include
+#  if __has_include(<fftw3.h>)
+#    include <fftw3.h>
+#  else
+#    error "This feature is unavailable without fftw3 installed"
+#endif
+#endif
+
 namespace boost { namespace math {
 
 namespace detail{

test/compile_test/interpolators_cardinal_trig_incl_test.cpp

@@ -6,6 +6,8 @@
 // A sanity check that this file
 // #includes all the files that it needs to.
 //
+#if __has_include(<fftw3.h>)
+
 #include <boost/math/interpolators/cardinal_trigonometric.hpp>
 //
 // Note this header includes no other headers, this is
@@ -23,3 +25,6 @@ void compile_and_link_test()
    boost::math::interpolators::cardinal_trigonometric<std::vector<double>> s(data, 3, 2);
    check_result<double>(s(1.0));
 }
+#else
+void compile_and_link_test() {}
+#endif // __has_include

test/compile_test/sf_chebyshev_transform_incl_test.cpp

@@ -6,6 +6,8 @@
 // Basic sanity check that header <boost/math/special_functions/chebyshev_transform.hpp>
 // #includes all the files that it needs to.
 //
+#if __has_include(<fftw3.h>)
+
 #include <boost/math/special_functions/chebyshev_transform.hpp>
 //
 // Note this header includes no other headers, this is
@@ -19,3 +21,7 @@ void compile_and_link_test()
     boost::math::chebyshev_transform<double> test(f, 0.0, 1.0);
     check_result<double>(test(1.0));
 }
+
+#else
+void compile_and_link_test() {}
+#endif // __has_include