"Fix" handling of CMAKE_Fortran_FLAGS*.

[dhermes]

TL;DR the set(OLD_CMAKE_Fortran_FLAGS... lines (and the corresponding unset(OLD_CMAKE_Fortran_FLAG... lines) should just be removed. Documentation should be added that explains how to use any of the following to customize the build:

• FFLAGS environment variable
• CMAKE_Fortran_FLAGS environment variable
• CMAKE_Fortran_FLAGS_DEBUG environment variable
• CMAKE_Fortran_FLAGS_RELEASE environment variable
• CMAKE_Fortran_FLAGS_<CMAKE_BUILD_TYPE> environment variable (if any CMAKE_BUILD_TYPE other than Debug or Release are added)
• -DCMAKE_Fortran_FLAGS flag to cmake
• -DCMAKE_Fortran_FLAGS_DEBUG flag to cmake
• -DCMAKE_Fortran_FLAGS_RELEASE flag to cmake
• -DCMAKE_Fortran_FLAGS_<CMAKE_BUILD_TYPE> flag to cmake

See also CMAKE_<LANG>_FLAGS, CMAKE_<LANG>_FLAGS_<CONFIG>, CMAKE_<LANG>_FLAGS_DEBUG, CMAKE_<LANG>_FLAGS_INIT, etc.

---

Right now, the "restore" stage just puts the flags back at the end of configure (so that they are present at build time).

For example (via conda-forge/staged-recipes#10762), running

cmake                                                       ^
    -G "MinGW Makefiles"                                    ^
    -DCMAKE_IGNORE_PATH="%CMAKE_IGNORE_PATH%"               ^
    -DCMAKE_Fortran_COMPILER=gfortran                       ^
    -DCMAKE_BUILD_TYPE=Release                              ^
    -DCMAKE_INSTALL_PREFIX:PATH="%LIBRARY_PREFIX%"          ^
    -DCMAKE_Fortran_FLAGS="-fno-asynchronous-unwind-tables" ^
    -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON                        ^
    -S "%SRC_DIR%\src\fortran"                              ^
    -B .

produces a build with the flag present:

...
%BUILD_PREFIX%\Library\mingw-w64\bin\gfortran.exe -Dbezier_EXPORTS  -fno-asynchronous-unwind-tables -O3 -DNDEBUG -O3   -fno-second-underscore -Wall -Wextra -Wno-compare-reals -Wimplicit-interface -fmax-errors=1 -std=f2008 -Werror -march=native -DNDEBUG -O3 -funroll-loops -c %SRC_DIR%\src\fortran\curve.f90 -o CMakeFiles\bezier.dir\curve.f90.obj
...

Note that this can be set either as a -DCMAKE_Fortran_FLAGS* flag, via the CMAKE_Fortran_FLAGS* environment variables or via the special FFLAGS environment variable.

---

We can get some "helpful" diagnostics on the values of these flags via

diff --git a/src/fortran/CMakeLists.txt b/src/fortran/CMakeLists.txt
index 82bcdfc4..af09ebcb 100644
--- a/src/fortran/CMakeLists.txt
+++ b/src/fortran/CMakeLists.txt
@@ -44,6 +44,10 @@ set(CMAKE_Fortran_FLAGS "")
 set(CMAKE_Fortran_FLAGS_DEBUG "")
 set(CMAKE_Fortran_FLAGS_RELEASE "")
 
+if(NOT "${OLD_CMAKE_Fortran_FLAGS}" STREQUAL "")
+  message(FATAL_ERROR "OLD_CMAKE_Fortran_FLAGS = ${OLD_CMAKE_Fortran_FLAGS}, CMAKE_Fortran_FLAGS = ${CMAKE_Fortran_FLAGS})")
+endif()
+
 add_subdirectory(quadpack)
 
 file(GLOB SOURCES *.f90)

and can ensure flags don't get through via:

diff --git a/src/fortran/CMakeLists.txt b/src/fortran/CMakeLists.txt
index 82bcdfc4..f8932d6e 100644
--- a/src/fortran/CMakeLists.txt
+++ b/src/fortran/CMakeLists.txt
@@ -104,11 +104,3 @@ install(
 install(DIRECTORY include/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
 install(EXPORT BezierConfig DESTINATION share/bezier/cmake)
 export(TARGETS bezier FILE BezierConfig.cmake)
-
-# Restore all Fortran flags.
-set(CMAKE_Fortran_FLAGS ${OLD_CMAKE_Fortran_FLAGS})
-set(CMAKE_Fortran_FLAGS_DEBUG ${OLD_CMAKE_Fortran_FLAGS_DEBUG})
-set(CMAKE_Fortran_FLAGS_RELEASE ${OLD_CMAKE_Fortran_FLAGS_RELEASE})
-unset(OLD_CMAKE_Fortran_FLAGS)
-unset(OLD_CMAKE_Fortran_FLAGS_DEBUG)
-unset(OLD_CMAKE_Fortran_FLAGS_RELEASE)