[SCons] Stop using header-only version of fmt library

While using the fmt library in header-only mode was convenient,
Cantera uses this library in so many separate source files that using
it in this mode was noticably increasing overall compilation time.

SConstruct

@@ -444,7 +444,12 @@ config_options = [
         'system_fmt',
         """Select whether to use the fmt library from a system installation
            ('y'), from a Git submodule ('n'), or to decide automatically
-           ('default').""",
+           ('default').  If you do not want to use the Git submodule and fmt
+           is not installed directly into system include and library
+           directories, then you will need to add those directories to
+           'extra_inc_dirs' and 'extra_lib_dirs'. This installation of fmt
+           must include the shared version of the library, for example,
+           'libfmt.so'.""",
         'default', ('default', 'y', 'n')),
     EnumVariable(
         'system_yamlcpp',
@@ -866,8 +871,9 @@ if not conf.CheckCXXHeader('cmath', '<>'):
     config_error('The C++ compiler is not correctly configured.')
 
 
-def get_expression_value(includes, expression):
-    s = ['#include ' + i for i in includes]
+def get_expression_value(includes, expression, defines=()):
+    s = ['#define ' + d for d in defines]
+    s.extend('#include ' + i for i in includes)
     s.extend(('#define Q(x) #x',
               '#define QUOTE(x) Q(x)',
               '#include <iostream>',
@@ -909,7 +915,7 @@ if env['system_fmt'] in ('n', 'default'):
                          '    git submodule update --init --recursive ext/fmt\n')
 
 fmt_include = '<fmt/format.h>' if env['system_fmt'] else '"../ext/fmt/include/fmt/format.h"'
-fmt_version_source = get_expression_value([fmt_include], 'FMT_VERSION')
+fmt_version_source = get_expression_value([fmt_include], 'FMT_VERSION', ['FMT_HEADER_ONLY'])
 retcode, fmt_lib_version = conf.TryRun(fmt_version_source, '.cpp')
 try:
     fmt_lib_version = divmod(float(fmt_lib_version.strip()), 10000)
@@ -1665,6 +1671,10 @@ else:
 linkLibs.extend(env['sundials_libs'])
 linkSharedLibs.extend(env['sundials_libs'])
 
+if env['system_fmt']:
+    linkLibs.append('fmt')
+    linkSharedLibs.append('fmt')
+
 if env['system_yamlcpp']:
     linkLibs.append('yaml-cpp')
     linkSharedLibs.append('yaml-cpp')

ext/SConscript

@@ -44,6 +44,10 @@ for subdir, extensions, prepFunction in libs:
 
 if not env['system_fmt']:
     license_files.append(('fmtlib', 'fmt/LICENSE.rst'))
+    localenv = prep_default(env)
+    localenv.Prepend(CPPPATH=Dir('#ext/fmt/include'))
+    libraryTargets.extend(
+        localenv.SharedObject(mglob(localenv, 'fmt/src', 'cc')))
     for name in ('format.h', 'ostream.h', 'printf.h', 'core.h', 'format-inl.h'):
         build(copyenv.Command("#include/cantera/ext/fmt/" + name,
                               "#ext/fmt/include/fmt/" + name,

include/cantera/base/fmt.h

@@ -7,10 +7,6 @@
 //! the same name in kinetics/Group.h
 #define FMT_NO_FMT_STRING_ALIAS
 
-//! Use header-only library to avoid relocation issues with linking to the
-//! static libfmt.a
-#define FMT_HEADER_ONLY
-
 //! Versions 6.2.0 and 6.2.1 of fmtlib do not include this define before they
 //! include windows.h, breaking builds on Windows. Fixed in fmtlib 7.0.0 and
 //! newer. https://github.com/fmtlib/fmt/pull/1616